Investments - Schedule of Unrealized Investment Losses by Class of Investment (Detail) $ in Thousands	Jun. 30, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	$ 3,196,745
Fair Value, Twelve Months or Longer	333,960
Fair Value, Total	3,530,705
Unrealized Loss Less Than Twelve Months	(197,544)
Unrealized Loss Twelve Months or Longer	(58,760)
Unrealized Loss Total	(256,304)
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	3,196,745
Fair Value, Twelve Months or Longer	333,960
Fair Value, Total	3,530,705
Unrealized Loss Less Than Twelve Months	(197,544)
Unrealized Loss Twelve Months or Longer	(58,760)
Unrealized Loss Total	(256,304)
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	0
Fair Value, Total	0
Unrealized Loss Less Than Twelve Months	0
Unrealized Loss Twelve Months or Longer	0
Unrealized Loss Total	0
External Credit Rating, Investment Grade [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	3,125,203
Fair Value, Twelve Months or Longer	123,538
Fair Value, Total	3,248,741
Unrealized Loss Less Than Twelve Months	(190,627)
Unrealized Loss Twelve Months or Longer	(18,101)
Unrealized Loss Total	(208,728)
External Credit Rating, Investment Grade [Member] | U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	305,538
Fair Value, Twelve Months or Longer	14,642
Fair Value, Total	320,180
Unrealized Loss Less Than Twelve Months	(16,522)
Unrealized Loss Twelve Months or Longer	(900)
Unrealized Loss Total	(17,422)
External Credit Rating, Investment Grade [Member] | States, Municipalities, and Political Subdivisions [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	79,844
Fair Value, Twelve Months or Longer	1,687
Fair Value, Total	81,531
Unrealized Loss Less Than Twelve Months	(1,803)
Unrealized Loss Twelve Months or Longer	(112)
Unrealized Loss Total	(1,915)
External Credit Rating, Investment Grade [Member] | Foreign Governments [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	4,134
Fair Value, Twelve Months or Longer	0
Fair Value, Total	4,134
Unrealized Loss Less Than Twelve Months	(379)
Unrealized Loss Twelve Months or Longer	0
Unrealized Loss Total	(379)
External Credit Rating, Investment Grade [Member] | Corporates [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	2,735,687
Fair Value, Twelve Months or Longer	105,564
Fair Value, Total	2,841,251
Unrealized Loss Less Than Twelve Months	(171,923)
Unrealized Loss Twelve Months or Longer	(17,067)
Unrealized Loss Total	(188,990)
External Credit Rating, Investment Grade [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	1,645
Fair Value, Total	1,645
Unrealized Loss Less Than Twelve Months	0
Unrealized Loss Twelve Months or Longer	(22)
Unrealized Loss Total	(22)
External Credit Rating, Investment Grade [Member] | Financial [Member] | Corporates [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	334,577
Fair Value, Twelve Months or Longer	0
Fair Value, Total	334,577
Unrealized Loss Less Than Twelve Months	(17,515)
Unrealized Loss Twelve Months or Longer	0
Unrealized Loss Total	(17,515)
External Credit Rating, Investment Grade [Member] | Utilities [Member] | Corporates [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	498,821
Fair Value, Twelve Months or Longer	0
Fair Value, Total	498,821
Unrealized Loss Less Than Twelve Months	(27,006)
Unrealized Loss Twelve Months or Longer	0
Unrealized Loss Total	(27,006)
External Credit Rating, Investment Grade [Member] | Utilities [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	1,645
Fair Value, Total	1,645
Unrealized Loss Less Than Twelve Months	0
Unrealized Loss Twelve Months or Longer	(22)
Unrealized Loss Total	(22)
External Credit Rating, Investment Grade [Member] | Energy [Member] | Corporates [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	372,105
Fair Value, Twelve Months or Longer	59,712
Fair Value, Total	431,817
Unrealized Loss Less Than Twelve Months	(30,440)
Unrealized Loss Twelve Months or Longer	(9,727)
Unrealized Loss Total	(40,167)
External Credit Rating, Investment Grade [Member] | Other Corporate Sectors [Member] | Corporates [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	1,530,184
Fair Value, Twelve Months or Longer	45,852
Fair Value, Total	1,576,036
Unrealized Loss Less Than Twelve Months	(96,962)
Unrealized Loss Twelve Months or Longer	(7,340)
Unrealized Loss Total	(104,302)
External Credit Rating, Non Investment Grade [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	71,542
Fair Value, Twelve Months or Longer	210,422
Fair Value, Total	281,964
Unrealized Loss Less Than Twelve Months	(6,917)
Unrealized Loss Twelve Months or Longer	(40,659)
Unrealized Loss Total	(47,576)
External Credit Rating, Non Investment Grade [Member] | States, Municipalities, and Political Subdivisions [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	319
Fair Value, Total	319
Unrealized Loss Less Than Twelve Months	0
Unrealized Loss Twelve Months or Longer	(236)
Unrealized Loss Total	(236)
External Credit Rating, Non Investment Grade [Member] | Corporates [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	71,542
Fair Value, Twelve Months or Longer	142,160
Fair Value, Total	213,702
Unrealized Loss Less Than Twelve Months	(6,917)
Unrealized Loss Twelve Months or Longer	(26,030)
Unrealized Loss Total	(32,947)
External Credit Rating, Non Investment Grade [Member] | Collateralized Debt Obligations [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	12,361
Fair Value, Total	12,361
Unrealized Loss Less Than Twelve Months	0
Unrealized Loss Twelve Months or Longer	(7,639)
Unrealized Loss Total	(7,639)
External Credit Rating, Non Investment Grade [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	55,582
Fair Value, Total	55,582
Unrealized Loss Less Than Twelve Months	0
Unrealized Loss Twelve Months or Longer	(6,754)
Unrealized Loss Total	(6,754)
External Credit Rating, Non Investment Grade [Member] | Financial [Member] | Corporates [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	84,947
Fair Value, Total	84,947
Unrealized Loss Less Than Twelve Months	0
Unrealized Loss Twelve Months or Longer	(20,851)
Unrealized Loss Total	(20,851)
External Credit Rating, Non Investment Grade [Member] | Financial [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	55,582
Fair Value, Total	55,582
Unrealized Loss Less Than Twelve Months	0
Unrealized Loss Twelve Months or Longer	(6,754)
Unrealized Loss Total	(6,754)
External Credit Rating, Non Investment Grade [Member] | Energy [Member] | Corporates [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	55,653
Fair Value, Twelve Months or Longer	0
Fair Value, Total	55,653
Unrealized Loss Less Than Twelve Months	(6,094)
Unrealized Loss Twelve Months or Longer	0
Unrealized Loss Total	(6,094)
External Credit Rating, Non Investment Grade [Member] | Other Corporate Sectors [Member] | Corporates [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	15,889
Fair Value, Twelve Months or Longer	57,213
Fair Value, Total	73,102
Unrealized Loss Less Than Twelve Months	(823)
Unrealized Loss Twelve Months or Longer	(5,179)
Unrealized Loss Total	$ (6,002)