Schedule of Unrealized Investment Losses by Class of Investment (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	$ 1,511,171
Fair Value, Twelve Months or Longer	391,592
Fair Value, Total	1,902,763
Unrealized Loss Less Than Twelve Months	(53,877)
Unrealized Loss Twelve Months or Longer	(57,511)
Unrealized Loss Total	(111,388)
U.S. Government Direct, Guaranteed, And Government-Sponsored Enterprises
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	301,179
Fair Value, Twelve Months or Longer	190
Fair Value, Total	301,369
Unrealized Loss Less Than Twelve Months	(12,668)
Unrealized Loss Twelve Months or Longer	(3)
Unrealized Loss Total	(12,671)
States, Municipalities, And Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	51,712
Fair Value, Twelve Months or Longer	0
Fair Value, Total	51,712
Unrealized Loss Less Than Twelve Months	(901)
Unrealized Loss Twelve Months or Longer	0
Unrealized Loss Total	(901)
Corporates
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	1,130,320
Fair Value, Twelve Months or Longer	267,190
Fair Value, Total	1,397,510
Unrealized Loss Less Than Twelve Months	(39,812)
Unrealized Loss Twelve Months or Longer	(30,066)
Unrealized Loss Total	(69,878)
Collateralized Debt Obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	49,989
Fair Value, Total	49,989
Unrealized Loss Less Than Twelve Months	0
Unrealized Loss Twelve Months or Longer	(15,199)
Unrealized Loss Total	(15,199)
Other Asset-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	7,850
Fair Value, Twelve Months or Longer	0
Fair Value, Total	7,850
Unrealized Loss Less Than Twelve Months	(47)
Unrealized Loss Twelve Months or Longer	0
Unrealized Loss Total	(47)
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	20,110
Fair Value, Twelve Months or Longer	74,223
Fair Value, Total	94,333
Unrealized Loss Less Than Twelve Months	(449)
Unrealized Loss Twelve Months or Longer	(12,243)
Unrealized Loss Total	(12,692)
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than Twelve Months	1,511,171
Fair Value, Twelve Months or Longer	391,592
Fair Value, Total	1,902,763
Unrealized Loss Less Than Twelve Months	(53,877)
Unrealized Loss Twelve Months or Longer	(57,511)
Unrealized Loss Total	(111,388)
Equity Securities
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