Organization and Basis of Presentation - Schedule of Adjustments to Consolidated Statement of Financial Condition and Consolidated Statements of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Error Corrections And Prior Period Adjustments Restatement [Line Items]
Total assets	$ 7,253,369		$ 7,253,369		$ 5,047,586
Total liabilities	6,320,489		6,320,489		3,361,469
Total equity	880,872		880,872		1,633,540	$ 1,586,535
Total liabilities, redeemable partnership interest, and equity	7,253,369		7,253,369		5,047,586
Income (loss) from operations before income taxes	142,402	$ 144,576	294,479	$ 233,193
Consolidated net income (loss)	110,548	114,303	239,395	187,475	253,838
Net income (loss) attributable to noncontrolling interest in subsidiaries	29,019	27,092	68,121	46,892
Net income (loss) available to common stockholders	$ 81,529	$ 87,211	$ 171,274	$ 140,583
Basic earnings (loss) per share	$ 0.28	$ 0.31	$ 0.60	$ 0.51
Diluted earnings (loss) per share	$ 0.28	$ 0.31	$ 0.59	$ 0.50
As Previously Reported [Member]
Error Corrections And Prior Period Adjustments Restatement [Line Items]
Total assets					3,508,400
Total liabilities					2,302,085
Total equity					1,153,738
Total liabilities, redeemable partnership interest, and equity					3,508,400
Income (loss) from operations before income taxes		$ 102,314		$ 150,908
Consolidated net income (loss)		72,051		105,257
Net income (loss) attributable to noncontrolling interest in subsidiaries		12,626		18,860
Net income (loss) available to common stockholders		$ 59,425		$ 86,397
Basic earnings (loss) per share		$ 0.21		$ 0.31
Diluted earnings (loss) per share		$ 0.21		$ 0.31
Retrospective Adjustments [Member]
Error Corrections And Prior Period Adjustments Restatement [Line Items]
Total assets					1,539,186
Total liabilities					1,059,384
Total equity					479,802
Total liabilities, redeemable partnership interest, and equity					$ 1,539,186
Income (loss) from operations before income taxes		$ 42,262		$ 82,285
Consolidated net income (loss)		42,252		82,218
Net income (loss) attributable to noncontrolling interest in subsidiaries		14,466		28,032
Net income (loss) available to common stockholders		$ 27,786		$ 54,186
Basic earnings (loss) per share		$ 0.10		$ 0.20
Diluted earnings (loss) per share		$ 0.10		$ 0.19