Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	$ 5,560,615	$ 6,364,591
Less Than 12 Months, Gross Unrealized Losses	350,573	590,647
Twelve Months or Longer, Fair Value	123,222	127,079
Twelve Months or Longer, Gross Unrealized Losses	7,682	12,683
Total, Fair Value	5,683,837	6,491,670
Total, Gross Unrealized Losses	358,255	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	4,460,561	4,254,477
Less Than 12 Months, Gross Unrealized Losses	335,998	536,114
Twelve Months or Longer, Fair Value	0	0
Twelve Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	4,460,561	4,254,477
Total, Gross Unrealized Losses	335,998	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	72,172	73,885
Less Than 12 Months, Gross Unrealized Losses	1,318	2,699
Twelve Months or Longer, Fair Value	0	0
Twelve Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	72,172	73,885
Total, Gross Unrealized Losses	1,318	2,699
Fixed maturities | Foreign government bonds
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	9,541	32,107
Less Than 12 Months, Gross Unrealized Losses	142	370
Twelve Months or Longer, Fair Value	0	0
Twelve Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	9,541	32,107
Total, Gross Unrealized Losses	142	370
Fixed maturities | Public utilities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	84,875	240,041
Less Than 12 Months, Gross Unrealized Losses	1,227	8,019
Twelve Months or Longer, Fair Value	13,863	17,097
Twelve Months or Longer, Gross Unrealized Losses	1,287	2,329
Total, Fair Value	98,738	257,138
Total, Gross Unrealized Losses	2,514	10,348
Fixed maturities | Redeemable preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	2,916	12,948
Less Than 12 Months, Gross Unrealized Losses	26	633
Twelve Months or Longer, Fair Value	0	0
Twelve Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	2,916	12,948
Total, Gross Unrealized Losses	26	633
Fixed maturities | All other U.S. public corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	309,507	530,904
Less Than 12 Months, Gross Unrealized Losses	2,906	8,798
Twelve Months or Longer, Fair Value	12,917	12,981
Twelve Months or Longer, Gross Unrealized Losses	965	1,355
Total, Fair Value	322,424	543,885
Total, Gross Unrealized Losses	3,871	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	156,813	453,976
Less Than 12 Months, Gross Unrealized Losses	2,500	13,632
Twelve Months or Longer, Fair Value	11,359	12,304
Twelve Months or Longer, Gross Unrealized Losses	522	178
Total, Fair Value	168,172	466,280
Total, Gross Unrealized Losses	3,022	13,810
Fixed maturities | All other foreign public corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	41,294	89,962
Less Than 12 Months, Gross Unrealized Losses	451	1,016
Twelve Months or Longer, Fair Value	0	9,994
Twelve Months or Longer, Gross Unrealized Losses	0	6
Total, Fair Value	41,294	99,956
Total, Gross Unrealized Losses	451	1,022
Fixed maturities | All other foreign private corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	69,025	247,111
Less Than 12 Months, Gross Unrealized Losses	1,141	11,661
Twelve Months or Longer, Fair Value	84,907	58,214
Twelve Months or Longer, Gross Unrealized Losses	4,905	8,789
Total, Fair Value	153,932	305,325
Total, Gross Unrealized Losses	6,046	20,450
Fixed maturities | Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	73,340	67,246
Less Than 12 Months, Gross Unrealized Losses	120	439
Twelve Months or Longer, Fair Value	176	16,489
Twelve Months or Longer, Gross Unrealized Losses	3	26
Total, Fair Value	73,516	83,735
Total, Gross Unrealized Losses	123	465
Fixed maturities | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	223,797	293,651
Less Than 12 Months, Gross Unrealized Losses	4,304	6,753
Twelve Months or Longer, Fair Value	0	0
Twelve Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	223,797	293,651
Total, Gross Unrealized Losses	4,304	6,753
Fixed maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less Than Twelve Months, Fair Value	56,774	68,283
Less Than 12 Months, Gross Unrealized Losses	440	513
Twelve Months or Longer, Fair Value	0	0
Twelve Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	56,774	68,283
Total, Gross Unrealized Losses	440	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