Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 2,990,738	$ 895,609
Less than 12 Months, Gross Unrealized Losses	89,100	7,733
Twelve Months or More, Fair Value	3,773,113	5,054,865
Twelve Months or More, Gross Unrealized Losses	371,621	247,973
Total, Fair Value	6,763,851	5,950,474
Total, Gross Unrealized Losses	460,721	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	186,994	13,174
Less than 12 Months, Gross Unrealized Losses	536	23
Twelve Months or More, Fair Value	3,338,339	4,550,472
Twelve Months or More, Gross Unrealized Losses	348,358	236,604
Total, Fair Value	3,525,333	4,563,646
Total, Gross Unrealized Losses	348,894	236,627
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	92,525	6,669
Less than 12 Months, Gross Unrealized Losses	1,842	26
Twelve Months or More, Fair Value	12,986	13,311
Twelve Months or More, Gross Unrealized Losses	429	132
Total, Fair Value	105,511	19,980
Total, Gross Unrealized Losses	2,271	158
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	147,070	37,466
Less than 12 Months, Gross Unrealized Losses	3,895	428
Twelve Months or More, Fair Value	2,952	143
Twelve Months or More, Gross Unrealized Losses	213	4
Total, Fair Value	150,022	37,609
Total, Gross Unrealized Losses	4,108	432
Public utilities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	314,310	84,260
Less than 12 Months, Gross Unrealized Losses	11,150	1,357
Twelve Months or More, Fair Value	24,570	22,420
Twelve Months or More, Gross Unrealized Losses	1,894	701
Total, Fair Value	338,880	106,680
Total, Gross Unrealized Losses	13,044	2,058
Redeemable preferred stock
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	28,826	10,522
Less than 12 Months, Gross Unrealized Losses	639	59
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	28,826	10,522
Total, Gross Unrealized Losses	639	59
All other U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	826,065	206,988
Less than 12 Months, Gross Unrealized Losses	32,893	1,034
Twelve Months or More, Fair Value	100,365	118,002
Twelve Months or More, Gross Unrealized Losses	4,235	2,382
Total, Fair Value	926,430	324,990
Total, Gross Unrealized Losses	37,128	3,416
All other U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	478,104	221,753
Less than 12 Months, Gross Unrealized Losses	13,884	2,173
Twelve Months or More, Fair Value	80,405	83,365
Twelve Months or More, Gross Unrealized Losses	4,992	1,622
Total, Fair Value	558,509	305,118
Total, Gross Unrealized Losses	18,876	3,795
All other foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	169,608	66,004
Less than 12 Months, Gross Unrealized Losses	4,824	578
Twelve Months or More, Fair Value	23,202	23,186
Twelve Months or More, Gross Unrealized Losses	850	340
Total, Fair Value	192,810	89,190
Total, Gross Unrealized Losses	5,674	918
All other foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	263,331	78,200
Less than 12 Months, Gross Unrealized Losses	9,240	536
Twelve Months or More, Fair Value	39,711	89,675
Twelve Months or More, Gross Unrealized Losses	3,218	2,695
Total, Fair Value	303,042	167,875
Total, Gross Unrealized Losses	12,458	3,231
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	195,728	30,234
Less than 12 Months, Gross Unrealized Losses	1,276	128
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	195,728	30,234
Total, Gross Unrealized Losses	1,276	128
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	238,172	113,423
Less than 12 Months, Gross Unrealized Losses	7,715	1,225
Twelve Months or More, Fair Value	125,108	129,458
Twelve Months or More, Gross Unrealized Losses	6,445	3,120
Total, Fair Value	363,280	242,881
Total, Gross Unrealized Losses	14,160	4,345
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	50,005	26,916
Less than 12 Months, Gross Unrealized Losses	1,206	166
Twelve Months or More, Fair Value	25,475	24,833
Twelve Months or More, Gross Unrealized Losses	987	373
Total, Fair Value	75,480	51,749
Total, Gross Unrealized Losses	$ 2,193	$ 539