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	$ 2,418,467	$ 6,364,591
Less Than 12 Months, Gross Unrealized Losses	80,217	590,647
Twelve Months or Longer, Fair Value	3,338,887	127,079
Twelve Months or Longer, Gross Unrealized Losses	281,341	12,683
Total, Fair Value	5,757,354	6,491,670
Total, Gross Unrealized Losses	361,558	603,330
Fixed maturities | U.S. 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	1,410,256	4,254,477
Less Than 12 Months, Gross Unrealized Losses	69,022	536,114
Twelve Months or Longer, Fair Value	3,045,887	0
Twelve Months or Longer, Gross Unrealized Losses	271,422	0
Total, Fair Value	4,456,143	4,254,477
Total, Gross Unrealized Losses	340,444	536,114
Fixed maturities | Obligations of U.S. states and their political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	58,449	73,885
Less Than 12 Months, Gross Unrealized Losses	641	2,699
Twelve Months or Longer, Fair Value	3,980	0
Twelve Months or Longer, Gross Unrealized Losses	110	0
Total, Fair Value	62,429	73,885
Total, Gross Unrealized Losses	751	2,699
Fixed maturities | Foreign government bonds
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	15,564	32,107
Less Than 12 Months, Gross Unrealized Losses	77	370
Twelve Months or Longer, Fair Value	145	0
Twelve Months or Longer, Gross Unrealized Losses	3	0
Total, Fair Value	15,709	32,107
Total, Gross Unrealized Losses	80	370
Fixed maturities | Public utilities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	106,170	240,041
Less Than 12 Months, Gross Unrealized Losses	1,592	8,019
Twelve Months or Longer, Fair Value	23,175	17,097
Twelve Months or Longer, Gross Unrealized Losses	1,372	2,329
Total, Fair Value	129,345	257,138
Total, Gross Unrealized Losses	2,964	10,348
Fixed maturities | Redeemable preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	0	12,948
Less Than 12 Months, Gross Unrealized Losses	0	633
Twelve Months or Longer, Fair Value	0	0
Twelve Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	0	12,948
Total, Gross Unrealized Losses	0	633
Fixed maturities | All other U.S. public corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	182,387	530,904
Less Than 12 Months, Gross Unrealized Losses	940	8,798
Twelve Months or Longer, Fair Value	83,448	12,981
Twelve Months or Longer, Gross Unrealized Losses	1,507	1,355
Total, Fair Value	265,835	543,885
Total, Gross Unrealized Losses	2,447	10,153
Fixed maturities | All other U.S. private corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	268,926	453,976
Less Than 12 Months, Gross Unrealized Losses	4,384	13,632
Twelve Months or Longer, Fair Value	41,675	12,304
Twelve Months or Longer, Gross Unrealized Losses	1,311	178
Total, Fair Value	310,601	466,280
Total, Gross Unrealized Losses	5,695	13,810
Fixed maturities | All other foreign public corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	64,229	89,962
Less Than 12 Months, Gross Unrealized Losses	452	1,016
Twelve Months or Longer, Fair Value	0	9,994
Twelve Months or Longer, Gross Unrealized Losses	0	6
Total, Fair Value	64,229	99,956
Total, Gross Unrealized Losses	452	1,022
Fixed maturities | All other foreign private corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	57,066	247,111
Less Than 12 Months, Gross Unrealized Losses	654	11,661
Twelve Months or Longer, Fair Value	78,062	58,214
Twelve Months or Longer, Gross Unrealized Losses	4,014	8,789
Total, Fair Value	135,128	305,325
Total, Gross Unrealized Losses	4,668	20,450
Fixed maturities | Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	36,356	67,246
Less Than 12 Months, Gross Unrealized Losses	50	439
Twelve Months or Longer, Fair Value	177	16,489
Twelve Months or Longer, Gross Unrealized Losses	3	26
Total, Fair Value	36,533	83,735
Total, Gross Unrealized Losses	53	465
Fixed maturities | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	181,908	293,651
Less Than 12 Months, Gross Unrealized Losses	2,131	6,753
Twelve Months or Longer, Fair Value	62,060	0
Twelve Months or Longer, Gross Unrealized Losses	1,589	0
Total, Fair Value	243,968	293,651
Total, Gross Unrealized Losses	3,720	6,753
Fixed maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	37,156	68,283
Less Than 12 Months, Gross Unrealized Losses	274	513
Twelve Months or Longer, Fair Value	278	0
Twelve Months or Longer, Gross Unrealized Losses	10	0
Total, Fair Value	37,434	68,283
Total, Gross Unrealized Losses	284	513
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	0	0
Less Than 12 Months, Gross Unrealized Losses	0	351
Twelve Months or Longer, Fair Value	0	0
Twelve Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	0	0
Total, Gross Unrealized Losses	$ 0	$ 351