Investments - Schedule of Unrealized Investment Losses by Class of Investment (Detail) $ in Thousands	Sep. 30, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	$ 3,258,114
Unrealized Loss, Less Than Twelve Months	(266,130)
Fair Value, Twelve Months or Longer	324,521
Unrealized Loss, Twelve Months or Longer	(96,080)
Fair Value, Total	3,582,635
Unrealized Loss Total	(362,210)
Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	3,258,114
Unrealized Loss, Less Than Twelve Months	(266,130)
Fair Value, Twelve Months or Longer	324,521
Unrealized Loss, Twelve Months or Longer	(96,080)
Fair Value, Total	3,582,635
Unrealized Loss Total	(362,210)
Equity securities [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	0
Unrealized Loss, Twelve Months or Longer	0
Fair Value, Total	0
Unrealized Loss Total	0
Investment grade [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	3,153,938
Unrealized Loss, Less Than Twelve Months	(235,002)
Fair Value, Twelve Months or Longer	153,557
Unrealized Loss, Twelve Months or Longer	(40,207)
Fair Value, Total	3,307,495
Unrealized Loss Total	(275,209)
Investment grade [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	216,022
Unrealized Loss, Less Than Twelve Months	(5,042)
Fair Value, Twelve Months or Longer	14,920
Unrealized Loss, Twelve Months or Longer	(654)
Fair Value, Total	230,942
Unrealized Loss Total	(5,696)
Investment grade [Member] | States, municipalities, and political subdivisions [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	48,097
Unrealized Loss, Less Than Twelve Months	(1,033)
Fair Value, Twelve Months or Longer	1,685
Unrealized Loss, Twelve Months or Longer	(107)
Fair Value, Total	49,782
Unrealized Loss Total	(1,140)
Investment grade [Member] | Foreign governments [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	4,574
Unrealized Loss, Less Than Twelve Months	(4)
Fair Value, Twelve Months or Longer	0
Unrealized Loss, Twelve Months or Longer	0
Fair Value, Total	4,574
Unrealized Loss Total	(4)
Investment grade [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	2,885,245
Unrealized Loss, Less Than Twelve Months	(228,923)
Fair Value, Twelve Months or Longer	136,952
Unrealized Loss, Twelve Months or Longer	(39,446)
Fair Value, Total	3,022,197
Unrealized Loss Total	(268,369)
Investment grade [Member] | Financial [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	283,220
Unrealized Loss, Less Than Twelve Months	(11,605)
Fair Value, Twelve Months or Longer	0
Unrealized Loss, Twelve Months or Longer	0
Fair Value, Total	283,220
Unrealized Loss Total	(11,605)
Investment grade [Member] | Utilities [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	403,693
Unrealized Loss, Less Than Twelve Months	(21,681)
Fair Value, Twelve Months or Longer	0
Unrealized Loss, Twelve Months or Longer	0
Fair Value, Total	403,693
Unrealized Loss Total	(21,681)
Investment grade [Member] | Energy [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	650,698
Unrealized Loss, Less Than Twelve Months	(69,271)
Fair Value, Twelve Months or Longer	92,025
Unrealized Loss, Twelve Months or Longer	(31,189)
Fair Value, Total	742,723
Unrealized Loss Total	(100,460)
Investment grade [Member] | Other corporate sectors [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	1,547,634
Unrealized Loss, Less Than Twelve Months	(126,366)
Fair Value, Twelve Months or Longer	44,927
Unrealized Loss, Twelve Months or Longer	(8,257)
Fair Value, Total	1,592,561
Unrealized Loss Total	(134,623)
Below investment grade [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	104,176
Unrealized Loss, Less Than Twelve Months	(31,128)
Fair Value, Twelve Months or Longer	170,964
Unrealized Loss, Twelve Months or Longer	(55,873)
Fair Value, Total	275,140
Unrealized Loss Total	(87,001)
Below investment grade [Member] | States, municipalities, and political subdivisions [Member] | Fixed maturities available for sale [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	321
Unrealized Loss, Twelve Months or Longer	(234)
Fair Value, Total	321
Unrealized Loss Total	(234)
Below investment grade [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	104,176
Unrealized Loss, Less Than Twelve Months	(31,128)
Fair Value, Twelve Months or Longer	136,506
Unrealized Loss, Twelve Months or Longer	(42,615)
Fair Value, Total	240,682
Unrealized Loss Total	(73,743)
Below investment grade [Member] | Collateralized debt obligations [Member] | Fixed maturities available for sale [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	11,407
Unrealized Loss, Twelve Months or Longer	(8,593)
Fair Value, Total	11,407
Unrealized Loss Total	(8,593)
Below investment grade [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [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	22,730
Unrealized Loss, Twelve Months or Longer	(4,431)
Fair Value, Total	22,730
Unrealized Loss Total	(4,431)
Below investment grade [Member] | Financial [Member] | Corporates [Member] | Fixed maturities available for sale [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	72,564
Unrealized Loss, Twelve Months or Longer	(33,229)
Fair Value, Total	72,564
Unrealized Loss Total	(33,229)
Below investment grade [Member] | Financial [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [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	22,730
Unrealized Loss, Twelve Months or Longer	(4,431)
Fair Value, Total	22,730
Unrealized Loss Total	(4,431)
Below investment grade [Member] | Energy [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	42,319
Unrealized Loss, Less Than Twelve Months	(8,391)
Fair Value, Twelve Months or Longer	8,106
Unrealized Loss, Twelve Months or Longer	(2,920)
Fair Value, Total	50,425
Unrealized Loss Total	(11,311)
Below investment grade [Member] | Other corporate sectors [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	61,857
Unrealized Loss, Less Than Twelve Months	(22,737)
Fair Value, Twelve Months or Longer	55,836
Unrealized Loss, Twelve Months or Longer	(6,466)
Fair Value, Total	117,693
Unrealized Loss Total	$ (29,203)