Investments (Securities Available for Sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt securities:
Total	$ 3,087,827	$ 2,964,510
Gross Unrealized Gains		87,446
Gross Unrealized Losses		(10,825)
Fair Value	3,084,789	3,041,131
Equity securities
Amortized Cost		12,890
Gross Unrealized Gains		5,708
Gross Unrealized Losses		(120)
Fair Value		18,478
Amortized Cost	3,087,827	2,977,400
Gross Unrealized Gains	40,487	93,154
Gross Unrealized Losses	(43,525)	(10,945)
Fair Value		3,059,609
Fair Value
Less than 12 Months	1,153,795
12 Months or Greater	291,615
Total	1,445,410
Unrealized Losses
12 Months or Greater	18,962
Less than 12 Months	24,563
Total	43,525
Fair Value, Equity [Abstract]
Less than 12 Months		206,345
12 Months or Greater		298,219
Total		504,564
Unrealized Losses, Equity and Total [Abstract]
Less than 12 Months		1,254
12 Months or Greater		9,691
Total		10,945
Debt Securities
Fair Value
Less than 12 Months		205,099
12 Months or Greater		297,930
Total		503,029
Unrealized Losses
12 Months or Greater		9,648
Less than 12 Months		1,177
Total		10,825
States and political subdivisions
Debt securities:
Total	574	575
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(25)	(29)
Fair Value	549	546
Fair Value
Less than 12 Months	549	546
12 Months or Greater	0	0
Total	549	546
Unrealized Losses
12 Months or Greater	0	0
Less than 12 Months	25	29
Total	25	29
Foreign governments
Debt securities:
Total	9,967	9,964
Gross Unrealized Gains	88	326
Gross Unrealized Losses	0	0
Fair Value	10,055	10,290
Public utilities
Debt securities:
Total	83,461	83,466
Gross Unrealized Gains	2,342	3,640
Gross Unrealized Losses	(128)	0
Fair Value	85,675	87,106
Fair Value
Less than 12 Months	17,841
12 Months or Greater	0
Total	17,841
Unrealized Losses
12 Months or Greater	0
Less than 12 Months	128
Total	128
Corporate
Debt securities:
Total	2,967,125	2,842,381
Gross Unrealized Gains	36,520	81,737
Gross Unrealized Losses	(43,336)	(10,744)
Fair Value	2,960,309	2,913,374
Fair Value
Less than 12 Months	1,133,923	201,575
12 Months or Greater	290,548	296,845
Total	1,424,471	498,420
Unrealized Losses
12 Months or Greater	18,926	9,610
Less than 12 Months	24,410	1,134
Total	43,336	10,744
Residential mortgage-backed
Debt securities:
Total	19,248	20,246
Gross Unrealized Gains	1,191	1,376
Gross Unrealized Losses	(36)	(52)
Fair Value	20,403	21,570
Fair Value
Less than 12 Months	0	1,325
12 Months or Greater	1,067	1,085
Total	1,067	2,410
Unrealized Losses
12 Months or Greater	36	38
Less than 12 Months	0	14
Total	36	52
Home equity
Debt securities:
Total	7,452	7,878
Gross Unrealized Gains	346	367
Gross Unrealized Losses	0	0
Fair Value	7,798	8,245
Fair Value
Less than 12 Months	1,482	1,653
12 Months or Greater	0	0
Total	1,482	1,653
Unrealized Losses
12 Months or Greater	0	0
Less than 12 Months	0	0
Total	$ 0	0
Manufactured housing
Debt securities:
Total		0
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		0
Equity securities
Fair Value, Equity [Abstract]
Less than 12 Months		1,246
12 Months or Greater		289
Total		1,535
Unrealized Losses, Equity and Total [Abstract]
Less than 12 Months		77
12 Months or Greater		43
Total		$ 120