Capitalized Servicing Assets - Changes in Capitalized Servicing Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Servicing Asset at Amortized Cost [Line Items]
Amortized value, Beginning balance	$ 221,115	$ 202,724	$ 186,917
Originations	63,412	68,735	65,470
Purchases	699	638	243
Amortization	(56,172)	(50,982)	(49,906)
Amortized value, ending balance	229,054	221,115	202,724
Ending balance, net	229,054	221,115	202,724
Residential Mortgage Loans [Member]
Servicing Asset at Amortized Cost [Line Items]
Amortized value, Beginning balance	117,351	118,303	109,871
Originations	28,792	28,618	35,556
Purchases	699	638	243
Amortization	(31,864)	(30,208)	(27,367)
Amortized value, ending balance	114,978	117,351	118,303
Ending balance, net	114,978	117,351	118,303
Commercial Mortgage Loans [Member]
Servicing Asset at Amortized Cost [Line Items]
Amortized value, Beginning balance	103,764	83,692	72,939
Originations	34,620	40,117	29,914
Amortization	(24,308)	(20,045)	(19,161)
Amortized value, ending balance	114,076	103,764	83,692
Ending balance, net	$ 114,076	103,764	83,692
Other [Member]
Servicing Asset at Amortized Cost [Line Items]
Amortized value, Beginning balance		729	4,107
Amortization		$ (729)	(3,378)
Amortized value, ending balance			729
Ending balance, net			$ 729