Loans Held for Sale - Composition of Loans Held for Sale (Detail) - USD ($)	9 Months Ended		13 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Dec. 31, 2015
Receivables Held-for-sale [Abstract]
Mortgage warehouse loans at fair value	$ 2,373,877,000		$ 2,373,877,000	$ 1,754,950,000
Multi-family loans at lower of cost or fair value	25,263,000		25,263,000	39,257,000
Commercial loans held for sale	2,399,140,000		2,399,140,000	1,794,207,000
Residential mortgage loans at fair value	3,568,000		3,568,000	2,857,000
Loans held for sale	$ 2,402,708,000		2,402,708,000	$ 1,797,064,000
Loans held for sale, average life from purchase to sale	20 days
Transfer of loans held for sale to held for investment	$ 0	$ 30,365,000	$ 0