Parent Company Only Condensed Financial Information Condensed Statement of Income (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 Condensed Consolidating Statement of Operations [Line Items]
Interest income	$ 405,016	$ 385,271	$ 353,927	$ 320,926	$ 307,333	$ 300,413	$ 293,992	$ 291,658	$ 1,465,140		$ 1,193,396	$ 1,046,765
Interest expense	(11,310)	(11,297)	(11,231)	(10,933)	(10,728)	(11,252)	(10,656)	(10,237)	(44,771)		(42,873)	(40,340)
Gains on derivative instruments, net									54,555		37,892	70,963
Gains on investment securities, net									64,603		51,740	89,445
Fund management fees									21,214		19,195	15,941
Income tax expense	(135,051)	(97,351)	(71,656)	(51,405)	(54,825)	(76,877)	(65,047)	(53,584)	(355,463)	[1]	(250,333)	(228,754)
Net income available to common stockholders	$ 117,210	$ 148,620	$ 123,193	$ 101,483	$ 99,466	$ 111,081	$ 92,964	$ 79,174	490,506	[1]	382,685	343,904
Parent Company
Schedule of Condensed Consolidating Statement of Operations [Line Items]
Interest income									2,077		690	964
Interest expense									(34,932)		(35,316)	(34,169)
Dividend income from subsidiary									90,000		40,000	0
Gains on derivative instruments, net									54,555		37,892	70,963
Gains on investment securities, net									37,132		20,644	39,447
Fund management fees									24,613		21,913	2,286
General and administrative expenses									(63,077)		(55,139)	(54,822)
Income tax expense									10,367		423	(14,448)
Loss before net income of subsidiaries									120,735		31,107	10,221
Net income available to common stockholders									490,506		382,685	343,904
Parent Company | Nonbank Subsidiaries
Schedule of Condensed Consolidating Statement of Operations [Line Items]
Loss before net income of subsidiaries									13,002		11,949	26,819
Equity in undistributed net income of subsidiary									13,002		11,949	26,819
Parent Company | Bank Subsidiaries
Schedule of Condensed Consolidating Statement of Operations [Line Items]
Loss before net income of subsidiaries									446,769		379,629	306,864
Equity in undistributed net income of subsidiary									$ 356,769		$ 339,629	$ 306,864

[1] Included in income tax expense, net income available to common stockholders, earnings per common share-basic and earnings for common share-diluted, for the year ended December 31, 2017, are tax benefits recognized associated with the adoption of Accounting Standards Update ("ASU") 2016-09, Improvements to Employee Share-Based Payment Accounting in the first quarter of 2017. This guidance was adopted on a prospective basis with no change to prior period amounts.