Available-For-Sale Securities (Available-For-Sale Securities, Continuous Unrealized Loss Position, Fair Value) (Detail) - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		$ 2,205,522	$ 1,807,913	$ 1,864,123
Continuous unrealized losses existing for less than 12 months, Fair value		1,109,505
Continuous unrealized losses existing for less than 12 months, Unrealized losses		(35,568)
Continuous unrealized losses existing for greater than 12 months, Fair value		469,835
Continuous unrealized losses existing for greater than 12 months, Unrealized losses		(26,380)
Total, Fair value		1,579,340
Total, Unrealized losses		(61,948)
U.S. Treasury
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		288,196	388,713	399,031
Continuous unrealized losses existing for less than 12 months, Fair value		207,997
Continuous unrealized losses existing for less than 12 months, Unrealized losses		(7,173)
Continuous unrealized losses existing for greater than 12 months, Fair value		0
Continuous unrealized losses existing for greater than 12 months, Unrealized losses		0
Total, Fair value		207,997
Total, Unrealized losses		(7,173)
U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		651,737	686,106	798,889
Continuous unrealized losses existing for less than 12 months, Fair value		231,514
Continuous unrealized losses existing for less than 12 months, Unrealized losses		(8,817)
Continuous unrealized losses existing for greater than 12 months, Fair value		248,487
Continuous unrealized losses existing for greater than 12 months, Unrealized losses		(16,334)
Total, Fair value		480,001
Total, Unrealized losses		(25,151)
Municipal
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		269,562	234,951	173,664
Continuous unrealized losses existing for less than 12 months, Fair value		96,407
Continuous unrealized losses existing for less than 12 months, Unrealized losses		(2,545)
Continuous unrealized losses existing for greater than 12 months, Fair value		37,578
Continuous unrealized losses existing for greater than 12 months, Unrealized losses		(1,449)
Total, Fair value		133,985
Total, Unrealized losses		(3,994)
Corporate notes, Financial issuers
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		124,924	129,309	129,211
Continuous unrealized losses existing for less than 12 months, Fair value		13,117
Continuous unrealized losses existing for less than 12 months, Unrealized losses		(94)
Continuous unrealized losses existing for greater than 12 months, Fair value		44,762
Continuous unrealized losses existing for greater than 12 months, Unrealized losses		(1,195)
Total, Fair value		57,879
Total, Unrealized losses		(1,289)
Corporate notes, Other
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		2,726	3,766	4,980
Continuous unrealized losses existing for less than 12 months, Fair value		998
Continuous unrealized losses existing for less than 12 months, Unrealized losses		(2)
Continuous unrealized losses existing for greater than 12 months, Fair value		0
Continuous unrealized losses existing for greater than 12 months, Unrealized losses		0
Total, Fair value		998
Total, Unrealized losses		(2)
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	[1]	777,087	271,129	255,082
Continuous unrealized losses existing for less than 12 months, Fair value		551,405
Continuous unrealized losses existing for less than 12 months, Unrealized losses		(16,869)
Continuous unrealized losses existing for greater than 12 months, Fair value		120,626
Continuous unrealized losses existing for greater than 12 months, Unrealized losses		(6,630)
Total, Fair value		672,031
Total, Unrealized losses		(23,499)
Mortgage-backed, Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	[1]	42,550	47,347	52,672
Continuous unrealized losses existing for less than 12 months, Fair value		5,158
Continuous unrealized losses existing for less than 12 months, Unrealized losses		(31)
Continuous unrealized losses existing for greater than 12 months, Fair value		9,877
Continuous unrealized losses existing for greater than 12 months, Unrealized losses		(401)
Total, Fair value		15,035
Total, Unrealized losses		(432)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		48,740	$ 46,592	$ 50,594
Continuous unrealized losses existing for less than 12 months, Fair value		2,909
Continuous unrealized losses existing for less than 12 months, Unrealized losses		(37)
Continuous unrealized losses existing for greater than 12 months, Fair value		8,505
Continuous unrealized losses existing for greater than 12 months, Unrealized losses		(371)
Total, Fair value		11,414
Total, Unrealized losses		$ (408)

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