Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 3,447,043	$ 895,609
Less than Twelve Months, Gross Unrealized Losses	105,665	7,733
Twelve Months or More, Fair Value	3,983,112	5,054,865
Twelve Months or More, Gross Unrealized Losses	522,216	247,973
Total, Fair Value	7,430,155	5,950,474
Total, Gross Unrealized Losses	627,881	255,706
U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	998,734	13,174
Less than Twelve Months, Gross Unrealized Losses	28,769	23
Twelve Months or More, Fair Value	3,194,749	4,550,472
Twelve Months or More, Gross Unrealized Losses	481,531	236,604
Total, Fair Value	4,193,483	4,563,646
Total, Gross Unrealized Losses	510,300	236,627
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	96,628	6,669
Less than Twelve Months, Gross Unrealized Losses	3,149	26
Twelve Months or More, Fair Value	19,062	13,311
Twelve Months or More, Gross Unrealized Losses	818	132
Total, Fair Value	115,690	19,980
Total, Gross Unrealized Losses	3,967	158
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	138,696	37,466
Less than Twelve Months, Gross Unrealized Losses	2,510	428
Twelve Months or More, Fair Value	23,115	143
Twelve Months or More, Gross Unrealized Losses	1,143	4
Total, Fair Value	161,811	37,609
Total, Gross Unrealized Losses	3,653	432
Public utilities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	284,042	84,260
Less than Twelve Months, Gross Unrealized Losses	12,460	1,357
Twelve Months or More, Fair Value	61,961	22,420
Twelve Months or More, Gross Unrealized Losses	3,662	701
Total, Fair Value	346,003	106,680
Total, Gross Unrealized Losses	16,122	2,058
Redeemable preferred stock
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	28,659	10,522
Less than Twelve Months, Gross Unrealized Losses	786	59
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	28,659	10,522
Total, Gross Unrealized Losses	786	59
All other U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	755,392	206,988
Less than Twelve Months, Gross Unrealized Losses	29,258	1,034
Twelve Months or More, Fair Value	134,461	118,002
Twelve Months or More, Gross Unrealized Losses	5,867	2,382
Total, Fair Value	889,853	324,990
Total, Gross Unrealized Losses	35,125	3,416
All other U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	351,239	221,753
Less than Twelve Months, Gross Unrealized Losses	9,807	2,173
Twelve Months or More, Fair Value	216,144	83,365
Twelve Months or More, Gross Unrealized Losses	10,858	1,622
Total, Fair Value	567,383	305,118
Total, Gross Unrealized Losses	20,665	3,795
All other foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	137,967	66,004
Less than Twelve Months, Gross Unrealized Losses	2,935	578
Twelve Months or More, Fair Value	52,986	23,186
Twelve Months or More, Gross Unrealized Losses	2,826	340
Total, Fair Value	190,953	89,190
Total, Gross Unrealized Losses	5,761	918
All other foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	223,148	78,200
Less than Twelve Months, Gross Unrealized Losses	10,928	536
Twelve Months or More, Fair Value	72,195	89,675
Twelve Months or More, Gross Unrealized Losses	5,710	2,695
Total, Fair Value	295,343	167,875
Total, Gross Unrealized Losses	16,638	3,231
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	268,933	30,234
Less than Twelve Months, Gross Unrealized Losses	1,539	128
Twelve Months or More, Fair Value	5,052	0
Twelve Months or More, Gross Unrealized Losses	146	0
Total, Fair Value	273,985	30,234
Total, Gross Unrealized Losses	1,685	128
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	137,333	113,423
Less than Twelve Months, Gross Unrealized Losses	2,773	1,225
Twelve Months or More, Fair Value	181,463	129,458
Twelve Months or More, Gross Unrealized Losses	8,703	3,120
Total, Fair Value	318,796	242,881
Total, Gross Unrealized Losses	11,476	4,345
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	26,272	26,916
Less than Twelve Months, Gross Unrealized Losses	751	166
Twelve Months or More, Fair Value	21,924	24,833
Twelve Months or More, Gross Unrealized Losses	952	373
Total, Fair Value	48,196	51,749
Total, Gross Unrealized Losses	$ 1,703	$ 539