Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	$ 490,171	$ 1,917,630
Less than twelve Months, Gross Unrealized Losses	6,929	68,678
Twelve months or more, Fair Value	358,115	284,359
Twelve months or more, Gross Unrealized Losses	19,482	33,798
Total, Fair Value	848,286	2,201,989
Total, Gross Unrealized Losses	26,411	102,476
Fixed maturities | US Treasury securities and obligations of U.S. government authorities and agencies
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	0	603
Less than twelve Months, Gross Unrealized Losses	0	33
Twelve months or more, Fair Value	0	0
Twelve months or more, Gross Unrealized Losses	0	0
Total, Fair Value	0	603
Total, Gross Unrealized Losses	0	33
Fixed maturities | US States and Political Subdivisions Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	20,622	239,146
Less than twelve Months, Gross Unrealized Losses	113	6,553
Twelve months or more, Fair Value	0	0
Twelve months or more, Gross Unrealized Losses	0	0
Total, Fair Value	20,622	239,146
Total, Gross Unrealized Losses	113	6,553
Fixed maturities | Foreign government bonds
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	7,077	81,074
Less than twelve Months, Gross Unrealized Losses	67	4,055
Twelve months or more, Fair Value	18,315	1,690
Twelve months or more, Gross Unrealized Losses	412	331
Total, Fair Value	25,392	82,764
Total, Gross Unrealized Losses	479	4,386
Fixed maturities | Public utilities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	35,073	207,226
Less than twelve Months, Gross Unrealized Losses	496	7,847
Twelve months or more, Fair Value	26,114	21,394
Twelve months or more, Gross Unrealized Losses	1,320	2,672
Total, Fair Value	61,187	228,620
Total, Gross Unrealized Losses	1,816	10,519
Fixed maturities | Redeemable preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	0	0
Less than twelve Months, Gross Unrealized Losses	0	0
Twelve months or more, Fair Value	0	0
Twelve months or more, Gross Unrealized Losses	0	156
Total, Fair Value	0	0
Total, Gross Unrealized Losses	0	156
Fixed maturities | All other U.S. public corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	55,600	568,763
Less than twelve Months, Gross Unrealized Losses	649	20,695
Twelve months or more, Fair Value	106,749	73,575
Twelve months or more, Gross Unrealized Losses	3,785	8,151
Total, Fair Value	162,349	642,338
Total, Gross Unrealized Losses	4,434	28,846
Fixed maturities | All other U.S. private corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	157,827	232,561
Less than twelve Months, Gross Unrealized Losses	2,782	6,082
Twelve months or more, Fair Value	29,476	29,071
Twelve months or more, Gross Unrealized Losses	892	1,620
Total, Fair Value	187,303	261,632
Total, Gross Unrealized Losses	3,674	7,702
Fixed maturities | All other foreign public corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	9,946	86,492
Less than twelve Months, Gross Unrealized Losses	48	3,188
Twelve months or more, Fair Value	13,976	5,433
Twelve months or more, Gross Unrealized Losses	633	939
Total, Fair Value	23,922	91,925
Total, Gross Unrealized Losses	681	4,127
Fixed maturities | All other foreign private corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	69,418	236,512
Less than twelve Months, Gross Unrealized Losses	1,279	13,604
Twelve months or more, Fair Value	130,224	101,858
Twelve months or more, Gross Unrealized Losses	11,474	19,851
Total, Fair Value	199,642	338,370
Total, Gross Unrealized Losses	12,753	33,455
Fixed maturities | Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	13,139	37,355
Less than twelve Months, Gross Unrealized Losses	6	492
Twelve months or more, Fair Value	363	49,346
Twelve months or more, Gross Unrealized Losses	3	70
Total, Fair Value	13,502	86,701
Total, Gross Unrealized Losses	9	562
Fixed maturities | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	110,027	191,674
Less than twelve Months, Gross Unrealized Losses	1,369	5,827
Twelve months or more, Fair Value	31,303	947
Twelve months or more, Gross Unrealized Losses	929	2
Total, Fair Value	141,330	192,621
Total, Gross Unrealized Losses	2,298	5,829
Fixed maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	11,442	36,224
Less than twelve Months, Gross Unrealized Losses	120	302
Twelve months or more, Fair Value	1,595	1,045
Twelve months or more, Gross Unrealized Losses	34	6
Total, Fair Value	13,037	37,269
Total, Gross Unrealized Losses	154	308
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	0	0
Less than twelve Months, Gross Unrealized Losses	0	0
Twelve months or more, Fair Value	0	2,965
Twelve months or more, Gross Unrealized Losses	0	152
Total, Fair Value	0	2,965
Total, Gross Unrealized Losses	$ 0	$ 152