Amortized Cost or Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		$ 16,285,600	$ 16,522,100
Gross Unrealized Gains		1,170,500	512,800
Gross Unrealized Losses		(57,100)	(163,800)
Fair Value		17,399,000	16,871,100
Equity securities, cost		3,170,673	2,816,572
Equity securities, gross unrealized gains		932,600	332,100
Equity securities, gross unrealized losses		(3,800)	(39,200)
Equity securities, fair value		4,099,467	3,109,523
Debt securities, amortized cost		12,536,772	12,927,103
Debt securities, gross unrealized gains		237,900	180,700
Debt securities, gross unrealized losses		(53,300)	(124,600)
Debt securities, fair value		12,721,399	12,983,213
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		963,900	1,265,700
Debt securities, gross unrealized gains		1,700	2,200
Debt securities, gross unrealized losses		(17,600)	(24,600)
Debt securities, fair value		948,000	1,243,300
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,578,900	4,161,000
Debt securities, gross unrealized gains		109,800	66,900
Debt securities, gross unrealized losses		(6,600)	(42,100)
Debt securities, fair value		3,682,100	4,185,800
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,000,100	1,030,900
Debt securities, gross unrealized gains		11,200	20,200
Debt securities, gross unrealized losses		(4,700)	(4,000)
Debt securities, fair value		1,006,600	1,047,100
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,381,100	2,168,900
Debt securities, gross unrealized gains		61,600	43,500
Debt securities, gross unrealized losses		(9,700)	(19,300)
Debt securities, fair value		2,433,000	2,193,100
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,481,800	1,068,300
Debt securities, gross unrealized gains		24,500	27,300
Debt securities, gross unrealized losses		(6,500)	(6,800)
Debt securities, fair value		1,499,800	1,088,800
Debt Securities | Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		993,900	1,005,900
Debt securities, gross unrealized gains		6,300	7,000
Debt securities, gross unrealized losses		(4,600)	(12,500)
Debt securities, fair value	[1]	995,600	1,000,400
Debt Securities | Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		545,000	728,800
Debt securities, gross unrealized gains		9,000	9,600
Debt securities, gross unrealized losses		(2,300)	(3,600)
Debt securities, fair value		551,700	734,800
Debt Securities | Mortgage and asset backed securities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[2]	1,592,100	1,497,600
Debt securities, gross unrealized gains	[2]	13,800	4,000
Debt securities, gross unrealized losses	[2]	(1,300)	(11,700)
Debt securities, fair value	[2]	1,604,600	1,489,900
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Equity securities, cost		3,165,800	2,816,600
Equity securities, gross unrealized gains		932,500	332,100
Equity securities, gross unrealized losses		(3,800)	(39,200)
Equity securities, fair value		4,094,500	3,109,500
Equity Securities | Preferred Stock
Schedule of Available-for-sale Securities [Line Items]
Equity securities, cost		4,900
Equity securities, gross unrealized gains		100
Equity securities, fair value		5,000
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		578,100	778,400
Fair Value		$ 578,100	$ 778,400

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

[2] Includes $1,101.3 million and $903.8 million of collateralized loan obligations as of December 31, 2017 and 2016, respectively.