Investments (Securities Available for Sale) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	$ 2,674,761	$ 2,577,275
Gross Unrealized Gains, Debt securities	167,900	143,234
Gross Unrealized Losses, Debt securities	(5,586)	(9,132)
Fair Value, Debt securities	2,837,075	2,711,377
Amortized Cost, Equity securities		12,799
Gross Unrealized Gains, Equity securities		4,849
Gross Unrealized Losses, Equity securities		(345)
Fair Value, Equity securities		17,303
Amortized Cost	2,687,568	2,590,074
Gross Unrealized Gains	172,864	148,083
Gross Unrealized Losses	(5,742)	(9,477)
Securities available for sale, at fair value (cost: $2,687,568 and $2,590,074)	2,854,690	2,728,680
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	104,836	100,678
Less than 12 Months, Unrealized Losses	(2,507)	(3,042)
12 Months or Greater, Fair Value	123,886	211,706
12 Months or Greater, Unrealized Losses	(3,235)	(6,435)
Total, Fair Value	228,722	312,384
Total, Unrealized Losses	(5,742)	(9,477)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, at fair value (cost: $2,687,568 and $2,590,074)	2,837,075	2,711,377
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	104,238	100,373
Less than 12 Months, Unrealized Losses	(2,401)	(2,990)
12 Months or Greater, Fair Value	122,341	207,905
12 Months or Greater, Unrealized Losses	(3,185)	(6,142)
Total, Fair Value	226,579	308,278
Total, Unrealized Losses	(5,586)	(9,132)
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	588	589
Gross Unrealized Gains, Debt securities	0	0
Gross Unrealized Losses, Debt securities	(35)	(36)
Fair Value, Debt securities	553	553
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	553	553
12 Months or Greater, Unrealized Losses	(35)	(36)
Total, Fair Value	553	553
Total, Unrealized Losses	(35)	(36)
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	9,941	9,939
Gross Unrealized Gains, Debt securities	684	386
Gross Unrealized Losses, Debt securities	0	0
Fair Value, Debt securities	10,625	10,325
Public Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	164,169	169,179
Gross Unrealized Gains, Debt securities	10,580	10,163
Gross Unrealized Losses, Debt securities	0	(126)
Fair Value, Debt securities	174,749	179,216
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	0	14,827
12 Months or Greater, Unrealized Losses	0	(126)
Total, Fair Value	0	14,827
Total, Unrealized Losses	0	(126)
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	2,441,317	2,334,700
Gross Unrealized Gains, Debt securities	152,193	128,280
Gross Unrealized Losses, Debt securities	(5,538)	(8,961)
Fair Value, Debt securities	2,587,972	2,454,019
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	104,238	100,373
Less than 12 Months, Unrealized Losses	(2,401)	(2,990)
12 Months or Greater, Fair Value	116,966	187,699
12 Months or Greater, Unrealized Losses	(3,137)	(5,971)
Total, Fair Value	221,204	288,072
Total, Unrealized Losses	(5,538)	(8,961)
Mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	45,563	48,674
Gross Unrealized Gains, Debt securities	4,177	4,116
Gross Unrealized Losses, Debt securities	0	0
Fair Value, Debt securities	49,740	52,790
Home Equity [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	11,577	11,702
Gross Unrealized Gains, Debt securities	213	225
Gross Unrealized Losses, Debt securities	(13)	(9)
Fair Value, Debt securities	11,777	11,918
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	4,822	4,826
12 Months or Greater, Unrealized Losses	(13)	(9)
Total, Fair Value	4,822	4,826
Total, Unrealized Losses	(13)	(9)
Manufactured Housing [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	1,606	2,492
Gross Unrealized Gains, Debt securities	53	64
Gross Unrealized Losses, Debt securities	0	0
Fair Value, Debt securities	1,659	2,556
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, at fair value (cost: $2,687,568 and $2,590,074)	17,615	17,303
Equity Public [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Equity securities	12,807
Gross Unrealized Gains, Equity securities	4,964
Gross Unrealized Losses, Equity securities	(156)
Fair Value, Equity securities	17,615
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	598	305
Less than 12 Months, Unrealized Losses	(106)	(52)
12 Months or Greater, Fair Value	1,545	3,801
12 Months or Greater, Unrealized Losses	(50)	(293)
Total, Fair Value	2,143	4,106
Total, Unrealized Losses	$ (156)	$ (345)