Amortized Cost or Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		$ 16,522,100	$ 16,636,700
Gross Unrealized Gains		512,800	573,800
Gross Unrealized Losses		(163,800)	(232,800)
Fair Value		16,871,100	16,977,700
Equity securities, cost		2,816,572	2,740,984
Equity securities, gross unrealized gains		332,100	351,900
Equity securities, gross unrealized losses		(39,200)	(87,000)
Equity securities, fair value		3,109,523	3,005,908
Debt securities, amortized cost		12,927,103	13,529,923
Debt securities, gross unrealized gains		180,700	221,900
Debt securities, gross unrealized losses		(124,600)	(145,800)
Debt securities, fair value		12,983,213	13,605,963
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,265,700	1,086,800
Debt securities, gross unrealized gains		2,200	1,900
Debt securities, gross unrealized losses		(24,600)	(14,000)
Debt securities, fair value		1,243,300	1,074,700
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		4,161,000	4,213,600
Debt securities, gross unrealized gains		66,900	134,800
Debt securities, gross unrealized losses		(42,100)	(8,800)
Debt securities, fair value		4,185,800	4,339,600
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,030,900	924,100
Debt securities, gross unrealized gains		20,200	18,600
Debt securities, gross unrealized losses		(4,000)	(1,300)
Debt securities, fair value		1,047,100	941,400
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,168,900	2,201,300
Debt securities, gross unrealized gains		43,500	23,400
Debt securities, gross unrealized losses		(19,300)	(48,000)
Debt securities, fair value		2,193,100	2,176,700
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,068,300	1,219,000
Debt securities, gross unrealized gains		27,300	24,000
Debt securities, gross unrealized losses		(6,800)	(12,700)
Debt securities, fair value		1,088,800	1,230,300
Debt Securities | Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,005,900	1,255,100
Debt securities, gross unrealized gains		7,000	10,700
Debt securities, gross unrealized losses		(12,500)	(12,400)
Debt securities, fair value	[1]	1,000,400	1,253,400
Debt Securities | Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		728,800	1,024,800
Debt securities, gross unrealized gains		9,600	8,200
Debt securities, gross unrealized losses		(3,600)	(9,600)
Debt securities, fair value		734,800	1,023,400
Debt Securities | Mortgage and asset backed securities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[2]	1,497,600	1,605,200
Debt securities, gross unrealized gains	[2]	4,000	300
Debt securities, gross unrealized losses	[2]	(11,700)	(39,000)
Debt securities, fair value	[2]	1,489,900	1,566,500
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Equity securities, cost		2,816,600	2,741,000
Equity securities, gross unrealized gains		332,100	351,900
Equity securities, gross unrealized losses		(39,200)	(87,000)
Equity securities, fair value		3,109,500	3,005,900
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		778,400	365,800
Fair Value		$ 778,400	$ 365,800

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

[2] Includes $903.8 million and $946.7 million of collateralized loan obligations as of December 31, 2016 and 2015, respectively.