Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 895,609	$ 6,364,591
Less than Twelve Months, Gross Unrealized Losses	7,733	590,647
Twelve Months or Longer, Fair Value	5,054,865	127,079
Twelve Months or Longer, Gross Unrealized Losses	247,973	12,683
Total, Fair Value	5,950,474	6,491,670
Total, Gross Unrealized Losses	255,706	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	13,174	4,254,477
Less than Twelve Months, Gross Unrealized Losses	23	536,114
Twelve Months or Longer, Fair Value	4,550,472	0
Twelve Months or Longer, Gross Unrealized Losses	236,604	0
Total, Fair Value	4,563,646	4,254,477
Total, Gross Unrealized Losses	236,627	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	6,669	73,885
Less than Twelve Months, Gross Unrealized Losses	26	2,699
Twelve Months or Longer, Fair Value	13,311	0
Twelve Months or Longer, Gross Unrealized Losses	132	0
Total, Fair Value	19,980	73,885
Total, Gross Unrealized Losses	158	2,699
Fixed maturities | Foreign government bonds
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	37,466	32,107
Less than Twelve Months, Gross Unrealized Losses	428	370
Twelve Months or Longer, Fair Value	143	0
Twelve Months or Longer, Gross Unrealized Losses	4	0
Total, Fair Value	37,609	32,107
Total, Gross Unrealized Losses	432	370
Fixed maturities | Public utilities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	84,260	240,041
Less than Twelve Months, Gross Unrealized Losses	1,357	8,019
Twelve Months or Longer, Fair Value	22,420	17,097
Twelve Months or Longer, Gross Unrealized Losses	701	2,329
Total, Fair Value	106,680	257,138
Total, Gross Unrealized Losses	2,058	10,348
Fixed maturities | Redeemable preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	10,522	12,948
Less than Twelve Months, Gross Unrealized Losses	59	633
Twelve Months or Longer, Fair Value	0	0
Twelve Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	10,522	12,948
Total, Gross Unrealized Losses	59	633
Fixed maturities | All other U.S. public corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	206,988	530,904
Less than Twelve Months, Gross Unrealized Losses	1,034	8,798
Twelve Months or Longer, Fair Value	118,002	12,981
Twelve Months or Longer, Gross Unrealized Losses	2,382	1,355
Total, Fair Value	324,990	543,885
Total, Gross Unrealized Losses	3,416	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	221,753	453,976
Less than Twelve Months, Gross Unrealized Losses	2,173	13,632
Twelve Months or Longer, Fair Value	83,365	12,304
Twelve Months or Longer, Gross Unrealized Losses	1,622	178
Total, Fair Value	305,118	466,280
Total, Gross Unrealized Losses	3,795	13,810
Fixed maturities | All other foreign public corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	66,004	89,962
Less than Twelve Months, Gross Unrealized Losses	578	1,016
Twelve Months or Longer, Fair Value	23,186	9,994
Twelve Months or Longer, Gross Unrealized Losses	340	6
Total, Fair Value	89,190	99,956
Total, Gross Unrealized Losses	918	1,022
Fixed maturities | All other foreign private corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	78,200	247,111
Less than Twelve Months, Gross Unrealized Losses	536	11,661
Twelve Months or Longer, Fair Value	89,675	58,214
Twelve Months or Longer, Gross Unrealized Losses	2,695	8,789
Total, Fair Value	167,875	305,325
Total, Gross Unrealized Losses	3,231	20,450
Fixed maturities | Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	30,234	67,246
Less than Twelve Months, Gross Unrealized Losses	128	439
Twelve Months or Longer, Fair Value	0	16,489
Twelve Months or Longer, Gross Unrealized Losses	0	26
Total, Fair Value	30,234	83,735
Total, Gross Unrealized Losses	128	465
Fixed maturities | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	113,423	293,651
Less than Twelve Months, Gross Unrealized Losses	1,225	6,753
Twelve Months or Longer, Fair Value	129,458	0
Twelve Months or Longer, Gross Unrealized Losses	3,120	0
Total, Fair Value	242,881	293,651
Total, Gross Unrealized Losses	4,345	6,753
Fixed maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	26,916	68,283
Less than Twelve Months, Gross Unrealized Losses	166	513
Twelve Months or Longer, Fair Value	24,833	0
Twelve Months or Longer, Gross Unrealized Losses	373	0
Total, Fair Value	51,749	68,283
Total, Gross Unrealized Losses	539	513
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	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