Annual report pursuant to Section 13 and 15(d)

Quarterly Financial Data (Unaudited) (Narrative) (Detail)

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Quarterly Financial Data (Unaudited) (Narrative) (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule of Quarterly Financial Information [Line Items]                      
Expenses $ 344,095 $ 317,649 $ 310,286 $ 294,961 $ 294,435 $ 281,162 $ 271,302 $ 272,124 $ 1,266,991 $ 1,119,023 $ 1,163,868
Non Cash Compensation                      
Schedule of Quarterly Financial Information [Line Items]                      
Expenses $ 1,391 $ 1,341 $ 1,378 $ 1,429 $ 1,315 $ 1,300 $ 1,341 $ 1,355