Amortized Cost or Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		$ 16,516,700	$ 16,636,700
Gross Unrealized Gains		634,700	573,800
Gross Unrealized Losses		(150,500)	(232,800)
Fair Value		17,000,900	16,977,700
Equity securities, cost		2,637,787	2,740,984
Equity securities, gross unrealized gains		307,700	351,900
Equity securities, gross unrealized losses		(56,400)	(87,000)
Equity securities, fair value		2,889,055	3,005,908
Debt securities, amortized cost		13,385,266	13,529,923
Debt securities, gross unrealized gains		327,000	221,900
Debt securities, gross unrealized losses		(94,100)	(145,800)
Debt securities, fair value		13,618,212	13,605,963
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		493,600	365,800
Fair Value		493,600	365,800
Mortgage and asset backed securities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[1]	1,585,700	1,605,200
Debt securities, gross unrealized gains	[1]	1,500	300
Debt securities, gross unrealized losses	[1]	(52,400)	(39,000)
Debt securities, fair value	[1]	1,534,800	1,566,500
Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,296,700	1,255,100
Debt securities, gross unrealized gains		18,800	10,700
Debt securities, gross unrealized losses		(2,000)	(12,400)
Debt securities, fair value	[2]	1,313,500	1,253,400
Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,009,700	1,024,800
Debt securities, gross unrealized gains		16,000	8,200
Debt securities, gross unrealized losses		(8,200)	(9,600)
Debt securities, fair value		1,017,500	1,023,400
Common Stock
Schedule of Available-for-sale Securities [Line Items]
Equity securities, cost		2,637,800	2,741,000
Equity securities, gross unrealized gains		307,700	351,900
Equity securities, gross unrealized losses		(56,400)	(87,000)
Equity securities, fair value		2,889,100	3,005,900
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,147,700	1,086,800
Debt securities, gross unrealized gains		19,200	1,900
Debt securities, gross unrealized losses		(700)	(14,000)
Debt securities, fair value		1,166,200	1,074,700
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		4,268,900	4,213,600
Debt securities, gross unrealized gains		159,400	134,800
Debt securities, gross unrealized losses		(8,400)	(8,800)
Debt securities, fair value		4,419,900	4,339,600
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,061,700	924,100
Debt securities, gross unrealized gains		29,900	18,600
Debt securities, gross unrealized losses		(200)	(1,300)
Debt securities, fair value		1,091,400	941,400
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,979,700	2,201,300
Debt securities, gross unrealized gains		50,200	23,400
Debt securities, gross unrealized losses		(18,500)	(48,000)
Debt securities, fair value		2,011,400	2,176,700
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,035,200	1,219,000
Debt securities, gross unrealized gains		32,000	24,000
Debt securities, gross unrealized losses		(3,700)	(12,700)
Debt securities, fair value		$ 1,063,500	$ 1,230,300

[1] Includes $913.9 million and $946.7 million of collateralized loan obligations as of March 31, 2016 and December 31, 2015, respectively.

[2] Primarily includes government agency pass-through securities guaranteed by a government agency or government sponsored enterprise, among other types of RMBS.