Investment Securities (Marketable Securities) (Detail) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 761,906	$ 1,731,292	$ 1,711,113
Gross Unrealized Gains		13,647	14,766	26,395
Gross Unrealized Losses		(4,570)	(29,670)	(16,478)
Available-for-sale securities, Fair Value		770,983	1,716,388	1,721,030
Held-to-maturity securities, Amortized Cost		911,715	884,826	0
Held-to-maturity securities, Gross Unrealized Gains		14,241	871	0
Held-to-maturity securities, Gross Unrealized Losses		(1,612)	(7,586)	0
Held-to-maturity securities, Fair Value		924,344	878,111	0
U.S. Treasury
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost		117,105	312,282	273,173
Gross Unrealized Gains		22	0	148
Gross Unrealized Losses		(38)	(5,553)	(1,847)
Available-for-sale securities, Fair Value		117,089	306,729	271,474
U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost		93,990	70,313	665,177
Gross Unrealized Gains		194	198	5,348
Gross Unrealized Losses		(12)	(275)	(8,732)
Available-for-sale securities, Fair Value		94,172	70,236	661,793
Held-to-maturity securities, Amortized Cost		712,732	687,302	0
Held-to-maturity securities, Gross Unrealized Gains		11,569	4	0
Held-to-maturity securities, Gross Unrealized Losses		(1,455)	(7,144)	0
Held-to-maturity securities, Fair Value		722,846	680,162	0
Municipal
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost		118,187	105,702	264,949
Gross Unrealized Gains		3,232	3,249	6,485
Gross Unrealized Losses		(224)	(356)	(1,522)
Available-for-sale securities, Fair Value		121,195	108,595	269,912
Held-to-maturity securities, Amortized Cost		198,983	197,524	0
Held-to-maturity securities, Gross Unrealized Gains		2,672	867	0
Held-to-maturity securities, Gross Unrealized Losses		(157)	(442)	0
Held-to-maturity securities, Fair Value		201,498	197,949	0
Corporate notes, Financial issuers
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost		78,048	80,014	129,360
Gross Unrealized Gains		1,492	1,510	1,965
Gross Unrealized Losses		(1,830)	(1,481)	(1,321)
Available-for-sale securities, Fair Value		77,710	80,043	130,004
Corporate notes, Other
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost		2,500	1,500	3,759
Gross Unrealized Gains		3	4	52
Gross Unrealized Losses		0	(2)	(1)
Available-for-sale securities, Fair Value		2,503	1,502	3,810
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	[1]	262,109	1,069,680	280,679
Gross Unrealized Gains	[1]	3,795	3,834	5,983
Gross Unrealized Losses	[1]	(1,900)	(21,004)	(2,529)
Available-for-sale securities, Fair Value	[1]	264,004	1,052,510	284,133
Mortgage-backed, Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	[1]	38,565	40,421	45,299
Gross Unrealized Gains	[1]	324	172	435
Gross Unrealized Losses	[1]	(198)	(506)	(276)
Available-for-sale securities, Fair Value	[1]	38,691	40,087	45,458
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost		51,402	51,380	48,717
Gross Unrealized Gains		4,585	5,799	5,979
Gross Unrealized Losses		(368)	(493)	(250)
Available-for-sale securities, Fair Value		55,619	56,686	54,446
Mortgage-backed securities, subprime
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value		$ 0	$ 0	$ 0

[1] Consisting entirely of residential mortgage-backed securities, none of which are subprime.