Investments - Schedule of Unrealized Investment Losses by Class of Investment (Details) - Fixed maturities available for sale [Member] $ in Thousands	Mar. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	$ 2,291,686
Unrealized Loss, Less Than Twelve Months	(70,411)
Fair Value, Twelve Months or Longer	629,278
Unrealized Loss, Twelve Months or Longer	(89,356)
Fair Value, Total	2,920,964
Unrealized Loss Total	(159,767)
Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	2,271,123
Unrealized Loss, Less Than Twelve Months	(70,288)
Fair Value, Twelve Months or Longer	311,155
Unrealized Loss, Twelve Months or Longer	(23,301)
Fair Value, Total	2,582,278
Unrealized Loss Total	(93,589)
Investment grade [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	243,425
Unrealized Loss, Less Than Twelve Months	(5,865)
Fair Value, Twelve Months or Longer	1,441
Unrealized Loss, Twelve Months or Longer	(559)
Fair Value, Total	244,866
Unrealized Loss Total	(6,424)
Investment grade [Member] | States, municipalities, and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	30,822
Unrealized Loss, Less Than Twelve Months	(779)
Fair Value, Twelve Months or Longer	668
Unrealized Loss, Twelve Months or Longer	(32)
Fair Value, Total	31,490
Unrealized Loss Total	(811)
Investment grade [Member] | Foreign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	2,187
Unrealized Loss, Less Than Twelve Months	(8)
Fair Value, Twelve Months or Longer	0
Unrealized Loss, Twelve Months or Longer	0
Fair Value, Total	2,187
Unrealized Loss Total	(8)
Investment grade [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	1,940,282
Unrealized Loss, Less Than Twelve Months	(63,023)
Fair Value, Twelve Months or Longer	309,046
Unrealized Loss, Twelve Months or Longer	(22,710)
Fair Value, Total	2,249,328
Unrealized Loss Total	(85,733)
Investment grade [Member] | Corporates [Member] | Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	335,653
Unrealized Loss, Less Than Twelve Months	(7,863)
Fair Value, Twelve Months or Longer	80,362
Unrealized Loss, Twelve Months or Longer	(3,632)
Fair Value, Total	416,015
Unrealized Loss Total	(11,495)
Investment grade [Member] | Corporates [Member] | Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	246,001
Unrealized Loss, Less Than Twelve Months	(7,937)
Fair Value, Twelve Months or Longer	16,675
Unrealized Loss, Twelve Months or Longer	(1,905)
Fair Value, Total	262,676
Unrealized Loss Total	(9,842)
Investment grade [Member] | Corporates [Member] | Energy [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	57,535
Unrealized Loss, Less Than Twelve Months	(955)
Fair Value, Twelve Months or Longer	141,685
Unrealized Loss, Twelve Months or Longer	(12,049)
Fair Value, Total	199,220
Unrealized Loss Total	(13,004)
Investment grade [Member] | Corporates [Member] | Other corporate sectors [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	1,301,093
Unrealized Loss, Less Than Twelve Months	(46,268)
Fair Value, Twelve Months or Longer	70,324
Unrealized Loss, Twelve Months or Longer	(5,124)
Fair Value, Total	1,371,417
Unrealized Loss Total	(51,392)
Investment grade [Member] | Other asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	24,580
Unrealized Loss, Less Than Twelve Months	(361)
Fair Value, Twelve Months or Longer	0
Unrealized Loss, Twelve Months or Longer	0
Fair Value, Total	24,580
Unrealized Loss Total	(361)
Investment grade [Member] | Redeemable preferred stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	29,827
Unrealized Loss, Less Than Twelve Months	(252)
Fair Value, Twelve Months or Longer	0
Unrealized Loss, Twelve Months or Longer	0
Fair Value, Total	29,827
Unrealized Loss Total	(252)
Investment grade [Member] | Redeemable preferred stocks [Member] | Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	23,985
Unrealized Loss, Less Than Twelve Months	(6)
Fair Value, Twelve Months or Longer	0
Unrealized Loss, Twelve Months or Longer	0
Fair Value, Total	23,985
Unrealized Loss Total	(6)
Investment grade [Member] | Redeemable preferred stocks [Member] | Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	5,842
Unrealized Loss, Less Than Twelve Months	(246)
Fair Value, Twelve Months or Longer	0
Unrealized Loss, Twelve Months or Longer	0
Fair Value, Total	5,842
Unrealized Loss Total	(246)
Below investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	20,563
Unrealized Loss, Less Than Twelve Months	(123)
Fair Value, Twelve Months or Longer	318,123
Unrealized Loss, Twelve Months or Longer	(66,055)
Fair Value, Total	338,686
Unrealized Loss Total	(66,178)
Below investment grade [Member] | States, municipalities, and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Unrealized Loss, Less Than Twelve Months	0
Fair Value, Twelve Months or Longer	333
Unrealized Loss, Twelve Months or Longer	(218)
Fair Value, Total	333
Unrealized Loss Total	(218)
Below investment grade [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	10,563
Unrealized Loss, Less Than Twelve Months	(123)
Fair Value, Twelve Months or Longer	287,320
Unrealized Loss, Twelve Months or Longer	(49,184)
Fair Value, Total	297,883
Unrealized Loss Total	(49,307)
Below investment grade [Member] | Corporates [Member] | Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Unrealized Loss, Less Than Twelve Months	0
Fair Value, Twelve Months or Longer	82,995
Unrealized Loss, Twelve Months or Longer	(22,765)
Fair Value, Total	82,995
Unrealized Loss Total	(22,765)
Below investment grade [Member] | Corporates [Member] | Energy [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	10,563
Unrealized Loss, Less Than Twelve Months	(123)
Fair Value, Twelve Months or Longer	88,907
Unrealized Loss, Twelve Months or Longer	(18,193)
Fair Value, Total	99,470
Unrealized Loss Total	(18,316)
Below investment grade [Member] | Corporates [Member] | Other corporate sectors [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Unrealized Loss, Less Than Twelve Months	0
Fair Value, Twelve Months or Longer	115,418
Unrealized Loss, Twelve Months or Longer	(8,226)
Fair Value, Total	115,418
Unrealized Loss Total	(8,226)
Below investment grade [Member] | Collateralized debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Unrealized Loss, Less Than Twelve Months	0
Fair Value, Twelve Months or Longer	10,351
Unrealized Loss, Twelve Months or Longer	(9,648)
Fair Value, Total	10,351
Unrealized Loss Total	(9,648)
Below investment grade [Member] | Redeemable preferred stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	10,000
Unrealized Loss, Less Than Twelve Months	0
Fair Value, Twelve Months or Longer	20,119
Unrealized Loss, Twelve Months or Longer	(7,005)
Fair Value, Total	30,119
Unrealized Loss Total	(7,005)
Below investment grade [Member] | Redeemable preferred stocks [Member] | Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	10,000
Unrealized Loss, Less Than Twelve Months	0
Fair Value, Twelve Months or Longer	20,119
Unrealized Loss, Twelve Months or Longer	(7,005)
Fair Value, Total	30,119
Unrealized Loss Total	$ (7,005)