Investments - Schedule of Unrealized Investment Losses by Class of Investment (Detail) - Fixed maturities available for sale [Member] $ in Thousands	Mar. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	$ 1,514,176
Unrealized Loss, Less Than Twelve Months	(159,096)
Fair Value, Twelve Months or Longer	1,177,730
Unrealized Loss, Twelve Months or Longer	(234,768)
Fair Value, Total	2,691,906
Unrealized Loss Total	(393,864)
Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	1,373,216
Unrealized Loss, Less Than Twelve Months	(129,839)
Fair Value, Twelve Months or Longer	888,511
Unrealized Loss, Twelve Months or Longer	(91,899)
Fair Value, Total	2,261,727
Unrealized Loss Total	(221,738)
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	1,653
Unrealized Loss, Less Than Twelve Months	(1)
Fair Value, Twelve Months or Longer	44,002
Unrealized Loss, Twelve Months or Longer	(1,421)
Fair Value, Total	45,655
Unrealized Loss Total	(1,422)
Investment grade [Member] | States, municipalities, and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	5,226
Unrealized Loss, Less Than Twelve Months	(14)
Fair Value, Twelve Months or Longer	11,843
Unrealized Loss, Twelve Months or Longer	(60)
Fair Value, Total	17,069
Unrealized Loss Total	(74)
Investment grade [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	1,342,772
Unrealized Loss, Less Than Twelve Months	(129,481)
Fair Value, Twelve Months or Longer	832,666
Unrealized Loss, Twelve Months or Longer	(90,418)
Fair Value, Total	2,175,438
Unrealized Loss Total	(219,899)
Investment grade [Member] | Redeemable preferred stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	23,565
Unrealized Loss, Less Than Twelve Months	(343)
Fair Value, Twelve Months or Longer	0
Unrealized Loss, Twelve Months or Longer	0
Fair Value, Total	23,565
Unrealized Loss Total	(343)
Investment grade [Member] | Financial [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	210,987
Unrealized Loss, Less Than Twelve Months	(9,112)
Fair Value, Twelve Months or Longer	103,332
Unrealized Loss, Twelve Months or Longer	(9,463)
Fair Value, Total	314,319
Unrealized Loss Total	(18,575)
Investment grade [Member] | Financial [Member] | Redeemable preferred stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	23,565
Unrealized Loss, Less Than Twelve Months	(343)
Fair Value, Twelve Months or Longer	0
Unrealized Loss, Twelve Months or Longer	0
Fair Value, Total	23,565
Unrealized Loss Total	(343)
Investment grade [Member] | Utilities [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	79,078
Unrealized Loss, Less Than Twelve Months	(10,368)
Fair Value, Twelve Months or Longer	90,495
Unrealized Loss, Twelve Months or Longer	(10,137)
Fair Value, Total	169,573
Unrealized Loss Total	(20,505)
Investment grade [Member] | Energy [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	581,465
Unrealized Loss, Less Than Twelve Months	(81,129)
Fair Value, Twelve Months or Longer	162,373
Unrealized Loss, Twelve Months or Longer	(45,263)
Fair Value, Total	743,838
Unrealized Loss Total	(126,392)
Investment grade [Member] | Other corporate sectors [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	471,242
Unrealized Loss, Less Than Twelve Months	(28,872)
Fair Value, Twelve Months or Longer	476,466
Unrealized Loss, Twelve Months or Longer	(25,555)
Fair Value, Total	947,708
Unrealized Loss Total	(54,427)
Below investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	140,960
Unrealized Loss, Less Than Twelve Months	(29,257)
Fair Value, Twelve Months or Longer	289,219
Unrealized Loss, Twelve Months or Longer	(142,869)
Fair Value, Total	430,179
Unrealized Loss Total	(172,126)
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	310
Unrealized Loss, Twelve Months or Longer	(244)
Fair Value, Total	310
Unrealized Loss Total	(244)
Below investment grade [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	140,960
Unrealized Loss, Less Than Twelve Months	(29,257)
Fair Value, Twelve Months or Longer	256,627
Unrealized Loss, Twelve Months or Longer	(127,758)
Fair Value, Total	397,587
Unrealized Loss Total	(157,015)
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	8,953
Unrealized Loss, Twelve Months or Longer	(11,047)
Fair Value, Total	8,953
Unrealized Loss Total	(11,047)
Below investment grade [Member] | Redeemable preferred stocks [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	23,329
Unrealized Loss, Twelve Months or Longer	(3,820)
Fair Value, Total	23,329
Unrealized Loss Total	(3,820)
Below investment grade [Member] | Financial [Member] | Corporates [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	63,652
Unrealized Loss, Twelve Months or Longer	(42,130)
Fair Value, Total	63,652
Unrealized Loss Total	(42,130)
Below investment grade [Member] | Financial [Member] | Redeemable preferred stocks [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	23,329
Unrealized Loss, Twelve Months or Longer	(3,820)
Fair Value, Total	23,329
Unrealized Loss Total	(3,820)
Below investment grade [Member] | Energy [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	27,193
Unrealized Loss, Less Than Twelve Months	(6,449)
Fair Value, Twelve Months or Longer	81,174
Unrealized Loss, Twelve Months or Longer	(49,283)
Fair Value, Total	108,367
Unrealized Loss Total	(55,732)
Below investment grade [Member] | Other corporate sectors [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	113,767
Unrealized Loss, Less Than Twelve Months	(22,808)
Fair Value, Twelve Months or Longer	111,801
Unrealized Loss, Twelve Months or Longer	(36,345)
Fair Value, Total	225,568
Unrealized Loss Total	$ (59,153)