Investments (Securities Available for Sale) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt securities:
Total	$ 3,008,624	$ 2,964,510
Debt securities, available for sale	2,946,059	3,041,131
Equity securities
Amortized Cost, Equity securities		12,890
Gross Unrealized Gains, Equity securities		5,708
Gross Unrealized Losses, Equity securities		(120)
Fair Value, Equity securities		18,478
Amortized Cost		2,977,400
Gross Unrealized Gains		93,154
Gross Unrealized Losses		(10,945)
Fair Value		3,059,609
Fair Value
Less than 12 Months	1,508,422
12 Months or Greater	443,676
Total	1,952,098
Unrealized Losses
Less than 12 Months	(45,096)
12 Months or Greater	(35,147)
Total	(80,243)
Fair Value [Abstract]
Less than 12 Months		298,219
12 Months or Greater		206,345
Total		504,564
Unrealized Losses [Abstract]
Less than 12 Months		(1,254)
12 Months or Greater		(9,691)
Total		(10,945)
Debt securities
Debt securities:
Total	3,008,624	2,964,510
Gross Unrealized Gains, Debt securities	17,678	87,446
Gross Unrealized Losses, Debt securities	(80,243)	(10,825)
Debt securities, available for sale	2,946,059	3,041,131
Fair Value [Abstract]
Less than 12 Months		297,930
12 Months or Greater		205,099
Total		503,029
Unrealized Losses [Abstract]
Less than 12 Months		(1,177)
12 Months or Greater		(9,648)
Total		(10,825)
States and political subdivisions
Debt securities:
Total	570	575
Gross Unrealized Gains, Debt securities	0	0
Gross Unrealized Losses, Debt securities	(4)	(29)
Debt securities, available for sale	566	546
Fair Value
Less than 12 Months	566
12 Months or Greater	0
Total	566
Unrealized Losses
Less than 12 Months	(4)
12 Months or Greater	0
Total	(4)
Fair Value [Abstract]
Less than 12 Months		0
12 Months or Greater		546
Total		546
Unrealized Losses [Abstract]
Less than 12 Months		(29)
12 Months or Greater		0
Total		(29)
Foreign governments
Debt securities:
Total	9,974	9,964
Gross Unrealized Gains, Debt securities	30	326
Gross Unrealized Losses, Debt securities	0	0
Debt securities, available for sale	10,004	10,290
Public utilities
Debt securities:
Total	82,943	83,466
Gross Unrealized Gains, Debt securities	1,045	3,640
Gross Unrealized Losses, Debt securities	(517)	0
Debt securities, available for sale	83,471	87,106
Fair Value
Less than 12 Months	38,903
12 Months or Greater	0
Total	38,903
Unrealized Losses
Less than 12 Months	(517)
12 Months or Greater	0
Total	(517)
Corporate
Debt securities:
Total	2,893,221	2,842,381
Gross Unrealized Gains, Debt securities	15,473	81,737
Gross Unrealized Losses, Debt securities	(79,638)	(10,744)
Debt securities, available for sale	2,829,056	2,913,374
Fair Value
Less than 12 Months	1,468,953
12 Months or Greater	442,798
Total	1,911,751
Unrealized Losses
Less than 12 Months	(44,575)
12 Months or Greater	(35,063)
Total	(79,638)
Fair Value [Abstract]
Less than 12 Months		296,845
12 Months or Greater		201,575
Total		498,420
Unrealized Losses [Abstract]
Less than 12 Months		(1,134)
12 Months or Greater		(9,610)
Total		(10,744)
Residential mortgage-backed
Debt securities:
Total	15,947	20,246
Gross Unrealized Gains, Debt securities	937	1,376
Gross Unrealized Losses, Debt securities	(84)	(52)
Debt securities, available for sale	16,800	21,570
Fair Value
Less than 12 Months	0
12 Months or Greater	878
Total	878
Unrealized Losses
Less than 12 Months	0
12 Months or Greater	(84)
Total	(84)
Fair Value [Abstract]
Less than 12 Months		1,085
12 Months or Greater		1,325
Total		2,410
Unrealized Losses [Abstract]
Less than 12 Months		(14)
12 Months or Greater		(38)
Total		(52)
Home equity
Debt securities:
Total	5,969	7,878
Gross Unrealized Gains, Debt securities	193	367
Gross Unrealized Losses, Debt securities	0	0
Debt securities, available for sale	6,162	8,245
Fair Value [Abstract]
Less than 12 Months		0
12 Months or Greater		1,653
Total		1,653
Unrealized Losses [Abstract]
Less than 12 Months		0
12 Months or Greater		0
Total		0
Manufactured housing
Debt securities:
Total	0	0
Gross Unrealized Gains, Debt securities	0	0
Gross Unrealized Losses, Debt securities	0	0
Debt securities, available for sale	$ 0	0
Equity securities
Fair Value [Abstract]
Less than 12 Months		289
12 Months or Greater		1,246
Total		1,535
Unrealized Losses [Abstract]
Less than 12 Months		(77)
12 Months or Greater		(43)
Total		$ (120)