Investments (Securities Available for Sale) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total	$ 2,928,818	$ 2,977,020
Gross Unrealized Gains, Debt securities	105,143	87,962
Gross Unrealized Losses, Debt securities	(10,821)	(22,932)
Fair Value, Debt securities	3,023,140	3,042,050
Amortized Cost, Equity securities	14,508	14,022
Gross Unrealized Gains, Equity securities	5,286	4,657
Gross Unrealized Losses, Equity securities	(176)	(366)
Fair Value, Equity securities	19,618	18,313
Amortized Cost	2,943,326	2,991,042
Gross Unrealized Gains	110,429	92,619
Gross Unrealized Losses	(10,997)	(23,298)
Fair Value	3,042,758	3,060,363
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	345,428	697,773
Less than 12 Months, Unrealized Losses	(6,919)	(17,061)
12 Months or Greater, Fair Value	91,372	107,391
12 Months or Greater, Unrealized Losses	(4,078)	(6,237)
Total, Fair Value	436,800	805,164
Total, Unrealized Losses	(10,997)	(23,298)
Due in 1 year or less	316,721
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	3,023,140	3,042,050
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	344,846	693,619
Less than 12 Months, Unrealized Losses	(6,825)	(16,756)
12 Months or Greater, Fair Value	90,958	106,969
12 Months or Greater, Unrealized Losses	(3,996)	(6,176)
Total, Fair Value	435,804	800,588
Total, Unrealized Losses	(10,821)	(22,932)
States and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Total	578	580
Gross Unrealized Gains, Debt securities	13	0
Gross Unrealized Losses, Debt securities	0	(6)
Fair Value, Debt securities	591	574
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	574
Less than 12 Months, Unrealized Losses	0	(6)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	0	574
Total, Unrealized Losses	0	(6)
Foreign governments
Schedule of Available-for-sale Securities [Line Items]
Total	9,960	9,956
Gross Unrealized Gains, Debt securities	427	380
Gross Unrealized Losses, Debt securities	0	0
Fair Value, Debt securities	10,387	10,336
Public utilities
Schedule of Available-for-sale Securities [Line Items]
Total	103,919	127,181
Gross Unrealized Gains, Debt securities	4,877	4,745
Gross Unrealized Losses, Debt securities	0	(232)
Fair Value, Debt securities	108,796	131,694
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	10,765
Less than 12 Months, Unrealized Losses	0	(232)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	0	10,765
Total, Unrealized Losses	0	(232)
Corporate
Schedule of Available-for-sale Securities [Line Items]
Total	2,782,502	2,802,852
Gross Unrealized Gains, Debt securities	97,657	80,414
Gross Unrealized Losses, Debt securities	(10,748)	(22,603)
Fair Value, Debt securities	2,869,411	2,860,663
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	344,846	680,988
Less than 12 Months, Unrealized Losses	(6,825)	(16,427)
12 Months or Greater, Fair Value	89,807	106,969
12 Months or Greater, Unrealized Losses	(3,923)	(6,176)
Total, Fair Value	434,653	787,957
Total, Unrealized Losses	(10,748)	(22,603)
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Total	23,278	27,110
Gross Unrealized Gains, Debt securities	1,911	2,137
Gross Unrealized Losses, Debt securities	(73)	(91)
Fair Value, Debt securities	25,116	29,156
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	1,292
Less than 12 Months, Unrealized Losses	0	(91)
12 Months or Greater, Fair Value	1,151	0
12 Months or Greater, Unrealized Losses	(73)	0
Total, Fair Value	1,151	1,292
Total, Unrealized Losses	(73)	(91)
Home equity
Schedule of Available-for-sale Securities [Line Items]
Total	8,581	9,341
Gross Unrealized Gains, Debt securities	258	286
Gross Unrealized Losses, Debt securities	0	0
Fair Value, Debt securities	8,839	9,627
Manufactured housing
Schedule of Available-for-sale Securities [Line Items]
Total	0	0
Gross Unrealized Gains, Debt securities	0	0
Gross Unrealized Losses, Debt securities	0	0
Fair Value, Debt securities	0	0
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	19,618	18,313
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	582	4,154
Less than 12 Months, Unrealized Losses	(94)	(305)
12 Months or Greater, Fair Value	414	422
12 Months or Greater, Unrealized Losses	(82)	(61)
Total, Fair Value	996	4,576
Total, Unrealized Losses	$ (176)	$ (366)