Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 190,512	$ 1,996,758
Less than Twelve Months, Gross Unrealized Losses	4,890	81,764
Twelve Months or More, Fair Value	447,537	947,390
Twelve Months or More, Gross Unrealized Losses	31,818	60,301
Total, Fair Value	638,049	2,944,148
Total, Gross Unrealized Losses	36,708	142,065
US Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [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	630
Twelve Months or More, Gross Unrealized Losses	0	6
Total, Fair Value	0	630
Total, Gross Unrealized Losses	0	6
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	0	124,776
Less than Twelve Months, Gross Unrealized Losses	0	1,571
Twelve Months or More, Fair Value	0	31,215
Twelve Months or More, Gross Unrealized Losses	0	780
Total, Fair Value	0	155,991
Total, Gross Unrealized Losses	0	2,351
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	0	77,055
Less than Twelve Months, Gross Unrealized Losses	0	3,184
Twelve Months or More, Fair Value	8,511	59,700
Twelve Months or More, Gross Unrealized Losses	14	5,015
Total, Fair Value	8,511	136,755
Total, Gross Unrealized Losses	14	8,199
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	9,171	784,916
Less than Twelve Months, Gross Unrealized Losses	69	37,635
Twelve Months or More, Fair Value	140,380	213,147
Twelve Months or More, Gross Unrealized Losses	5,557	16,766
Total, Fair Value	149,551	998,063
Total, Gross Unrealized Losses	5,626	54,401
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	24,292	263,934
Less than Twelve Months, Gross Unrealized Losses	3,547	9,159
Twelve Months or More, Fair Value	54,624	287,031
Twelve Months or More, Gross Unrealized Losses	2,278	9,432
Total, Fair Value	78,916	550,965
Total, Gross Unrealized Losses	5,825	18,591
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	815	124,764
Less than Twelve Months, Gross Unrealized Losses	0	6,286
Twelve Months or More, Fair Value	33,148	72,725
Twelve Months or More, Gross Unrealized Losses	1,955	5,865
Total, Fair Value	33,963	197,489
Total, Gross Unrealized Losses	1,955	12,151
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	80,817	424,921
Less than Twelve Months, Gross Unrealized Losses	1,041	22,605
Twelve Months or More, Fair Value	184,542	127,201
Twelve Months or More, Gross Unrealized Losses	21,880	17,417
Total, Fair Value	265,359	552,122
Total, Gross Unrealized Losses	22,921	40,022
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	75,417	112,527
Less than Twelve Months, Gross Unrealized Losses	233	650
Twelve Months or More, Fair Value	15,424	6,523
Twelve Months or More, Gross Unrealized Losses	54	100
Total, Fair Value	90,841	119,050
Total, Gross Unrealized Losses	287	750
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	0	49,616
Less than Twelve Months, Gross Unrealized Losses	0	434
Twelve Months or More, Fair Value	2,961	116,786
Twelve Months or More, Gross Unrealized Losses	10	4,093
Total, Fair Value	2,961	166,402
Total, Gross Unrealized Losses	10	4,527
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	0	34,249
Less than Twelve Months, Gross Unrealized Losses	0	240
Twelve Months or More, Fair Value	7,947	32,432
Twelve Months or More, Gross Unrealized Losses	70	827
Total, Fair Value	7,947	66,681
Total, Gross Unrealized Losses	$ 70	$ 1,067