Available-For-Sale Securities (Schedule of Available-for-Sale Securities Reconciliation) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,711,113		$ 1,807,913		$ 2,015,966
Gross Unrealized Gains	26,395		22,145		17,212
Gross Unrealized Losses	(16,478)		(37,980)		(83,481)
Fair Value	1,721,030		1,792,078		1,949,697
U.S. Treasury
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	273,173		388,713		354,109
Gross Unrealized Gains	148		84		263
Gross Unrealized Losses	(1,847)		(6,992)		(14,194)
Fair Value	271,474		381,805		340,178
U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	665,177		686,106		874,845
Gross Unrealized Gains	5,348		4,113		3,286
Gross Unrealized Losses	(8,732)		(21,903)		(49,856)
Fair Value	661,793		668,316		828,275
Municipal
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	264,949		234,951		175,028
Gross Unrealized Gains	6,485		5,318		3,439
Gross Unrealized Losses	(1,522)		(1,740)		(3,167)
Fair Value	269,912		238,529		175,300
Corporate notes, Financial issuers
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	129,360		129,309		129,413
Gross Unrealized Gains	1,965		2,006		2,306
Gross Unrealized Losses	(1,321)		(1,557)		(1,735)
Fair Value	130,004		129,758		129,984
Corporate notes, Other
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,759		3,766		4,986
Gross Unrealized Gains	52		55		100
Gross Unrealized Losses	(1)		0		(3)
Fair Value	3,810		3,821		5,083
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	280,679	[1]	271,129	[1]	371,825	[1]
Gross Unrealized Gains	5,983	[1]	5,448	[1]	3,919	[1]
Gross Unrealized Losses	(2,529)	[1]	(4,928)	[1]	(13,188)	[1]
Fair Value	284,133	[1]	271,649	[1]	362,556	[1]
Mortgage-backed, Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,299	[1]	47,347	[1]	55,190	[1]
Gross Unrealized Gains	435	[1]	249	[1]	356	[1]
Gross Unrealized Losses	(276)	[1]	(535)	[1]	(799)	[1]
Fair Value	45,458	[1]	47,061	[1]	54,747	[1]
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,717		46,592		50,570
Gross Unrealized Gains	5,979		4,872		3,543
Gross Unrealized Losses	(250)		(325)		(539)
Fair Value	$ 54,446		$ 51,139		$ 53,574

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