Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 6,364,591	$ 592,004
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	590,647	15,922
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	127,079	160,128
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	12,683	18,791
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	6,491,670	752,132
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	603,330	34,713
U.S. Treasury securities and obligations of U.S. government authorities and agencies
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	4,254,477	8,480
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	536,114	107
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4,254,477	8,480
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	536,114	107
Obligations of U.S. states and their political subdivisions
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	73,885	6,887
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	2,699	378
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	73,885	6,887
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2,699	378
Foreign government bonds
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	32,107	13,616
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	370	28
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	32,107	13,616
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	370	28
Public utilities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	240,041	49,104
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	8,019	1,421
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	17,097	14,217
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	2,329	2,842
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	257,138	63,321
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	10,348	4,263
Redeemable Preferred Stock
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	12,948	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	633	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	12,948	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	633	0
All other U.S. public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	530,904	207,578
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	8,798	6,297
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	12,981	29,828
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	1,355	1,420
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	543,885	237,406
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	10,153	7,717
All other U.S. private corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	453,976	84,318
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	13,632	4,020
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	12,304	3,550
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	178	387
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	466,280	87,868
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	13,810	4,407
All other foreign public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	89,962	76,573
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	1,016	608
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	9,994	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	6	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	99,956	76,573
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1,022	608
All other foreign private corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	247,111	38,047
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	11,661	1,972
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	58,214	85,341
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	8,789	13,870
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	305,325	123,388
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	20,450	15,842
Asset-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	67,246	50,195
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	439	430
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	16,489	26,359
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	26	262
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	83,735	76,554
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	465	692
Commercial mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	293,651	55,065
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	6,753	642
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	833
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	0	10
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	293,651	55,898
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	6,753	652
Residential mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	68,283	2,141
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	513	19
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	68,283	2,141
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	513	19
Equity securities, available-for-sale
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Accumulated Loss	351	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 351	$ 0