Available-For-Sale Securities (Schedule of Available-for-Sale Securities Reconciliation) (Detail) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 2,226,376	$ 1,807,913	$ 1,821,033
Gross Unrealized Gains		20,153	22,145	21,156
Gross Unrealized Losses		(32,248)	(37,980)	(59,541)
Available-for-sale Securities		2,214,281	1,792,078	1,782,648
U.S. Treasury
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		288,185	388,713	388,873
Gross Unrealized Gains		101	84	372
Gross Unrealized Losses		(2,364)	(6,992)	(10,984)
Available-for-sale Securities		285,922	381,805	378,261
U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		657,297	686,106	771,255
Gross Unrealized Gains		2,726	4,113	3,866
Gross Unrealized Losses		(15,000)	(21,903)	(35,369)
Available-for-sale Securities		645,023	668,316	739,752
Municipal
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		294,073	234,951	184,015
Gross Unrealized Gains		5,354	5,318	4,969
Gross Unrealized Losses		(2,085)	(1,740)	(1,881)
Available-for-sale Securities		297,342	238,529	187,103
Corporate notes, Financial issuers
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		114,976	129,309	129,259
Gross Unrealized Gains		1,656	2,006	2,252
Gross Unrealized Losses		(1,216)	(1,557)	(1,208)
Available-for-sale Securities		115,416	129,758	130,303
Corporate notes, Other
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,525	3,766	3,773
Gross Unrealized Gains		6	55	79
Gross Unrealized Losses		(2)	0	0
Available-for-sale Securities		1,529	3,821	3,852
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[1]	778,240	271,129	246,354
Gross Unrealized Gains	[1]	4,974	5,448	4,303
Gross Unrealized Losses	[1]	(10,913)	(4,928)	(8,938)
Available-for-sale Securities	[1]	772,301	271,649	241,719
Mortgage-backed, Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[1]	42,724	47,347	49,909
Gross Unrealized Gains	[1]	343	249	357
Gross Unrealized Losses	[1]	(323)	(535)	(763)
Available-for-sale Securities	[1]	42,744	47,061	49,503
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		49,356	46,592	47,595
Gross Unrealized Gains		4,993	4,872	4,958
Gross Unrealized Losses		(345)	(325)	(398)
Available-for-sale Securities		54,004	51,139	52,155
Mortgage-backed securities, subprime
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities		$ 0	$ 0	$ 0

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