Schedule of Unrealized Investment Losses by Class of Investment (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	$ 1,156,351	$ 791,379
Fair Value, Twelve Months or Longer	570,465	1,034,873
Fair Value, Total	1,726,816	1,826,252
Unrealized Loss Less Than Twelve Months	(20,696)	(52,565)
Unrealized Loss Twelve Months or Longer	(68,345)	(186,669)
Unrealized Loss Total	(89,041)	(239,234)
U.S. Government Direct, Guaranteed, And Government-Sponsored Enterprises
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	316,596	279
Fair Value, Twelve Months or Longer	199	34
Fair Value, Total	316,795	313
Unrealized Loss Less Than Twelve Months	(4,770)	(3)
Unrealized Loss Twelve Months or Longer	(3)	(1)
Unrealized Loss Total	(4,773)	(4)
States, Municipalities, And Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	26,206	0
Fair Value, Twelve Months or Longer	0	17,609
Fair Value, Total	26,206	17,609
Unrealized Loss Less Than Twelve Months	(189)	0
Unrealized Loss Twelve Months or Longer	0	(1,896)
Unrealized Loss Total	(189)	(1,896)
Foreign Governments
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	0	0
Fair Value, Total	0	0
Unrealized Loss Less Than Twelve Months	0	0
Unrealized Loss Twelve Months or Longer	0	0
Unrealized Loss Total	0	0
Corporates
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	761,477	585,265
Fair Value, Twelve Months or Longer	343,987	612,338
Fair Value, Total	1,105,464	1,197,603
Unrealized Loss Less Than Twelve Months	(15,339)	(38,249)
Unrealized Loss Twelve Months or Longer	(39,684)	(99,659)
Unrealized Loss Total	(55,023)	(137,908)
Collateralized Debt Obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	46,446	30,320
Fair Value, Total	46,446	30,320
Unrealized Loss Less Than Twelve Months	0	0
Unrealized Loss Twelve Months or Longer	(18,051)	(30,117)
Unrealized Loss Total	(18,051)	(30,117)
Other Asset-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	7,940	0
Fair Value, Twelve Months or Longer	7,981	7,122
Fair Value, Total	15,921	7,122
Unrealized Loss Less Than Twelve Months	(88)	0
Unrealized Loss Twelve Months or Longer	(313)	(1,392)
Unrealized Loss Total	(401)	(1,392)
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	44,132	205,449
Fair Value, Twelve Months or Longer	171,852	367,450
Fair Value, Total	215,984	572,899
Unrealized Loss Less Than Twelve Months	(310)	(14,250)
Unrealized Loss Twelve Months or Longer	(10,294)	(53,604)
Unrealized Loss Total	(10,604)	(67,854)
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	1,156,351	790,993
Fair Value, Twelve Months or Longer	570,465	1,034,873
Fair Value, Total	1,726,816	1,825,866
Unrealized Loss Less Than Twelve Months	(20,696)	(52,502)
Unrealized Loss Twelve Months or Longer	(68,345)	(186,669)
Unrealized Loss Total	(89,041)	(239,171)
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0	386
Fair Value, Twelve Months or Longer	0	0
Fair Value, Total	0	386
Unrealized Loss Less Than Twelve Months	0	(63)
Unrealized Loss Twelve Months or Longer	0	0
Unrealized Loss Total	$ 0	$ (63)