Amortized Cost or Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		$ 12,676,900	$ 16,285,600
Gross Unrealized Gains		130,700	1,170,500
Gross Unrealized Losses		(129,700)	(57,100)
Fair Value		12,677,900	17,399,000
Equity securities, cost		3,721,932	3,170,673
Equity securities, gross unrealized gains			932,600
Equity securities, gross unrealized losses			(3,800)
Equity securities, fair value		4,545,899	4,099,467
Debt securities, amortized cost		12,065,159	12,536,772
Debt securities, gross unrealized gains		130,700	237,900
Debt securities, gross unrealized losses		(129,700)	(53,300)
Debt securities, fair value		12,066,223	12,721,399
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,060,400	963,900
Debt securities, gross unrealized gains		1,000	1,700
Debt securities, gross unrealized losses		(29,700)	(17,600)
Debt securities, fair value		1,031,700	948,000
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,101,200	3,578,900
Debt securities, gross unrealized gains		58,200	109,800
Debt securities, gross unrealized losses		(18,800)	(6,600)
Debt securities, fair value		3,140,600	3,682,100
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		954,600	1,000,100
Debt securities, gross unrealized gains		9,600	11,200
Debt securities, gross unrealized losses		(6,100)	(4,700)
Debt securities, fair value		958,100	1,006,600
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,275,400	2,381,100
Debt securities, gross unrealized gains		29,500	61,600
Debt securities, gross unrealized losses		(29,200)	(9,700)
Debt securities, fair value		2,275,700	2,433,000
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,416,500	1,481,800
Debt securities, gross unrealized gains		15,400	24,500
Debt securities, gross unrealized losses		(15,600)	(6,500)
Debt securities, fair value		1,416,300	1,499,800
Debt Securities | Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,077,000	993,900
Debt securities, gross unrealized gains		3,900	6,300
Debt securities, gross unrealized losses		(21,700)	(4,600)
Debt securities, fair value	[1]	1,059,200	995,600
Debt Securities | Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		526,400	545,000
Debt securities, gross unrealized gains		3,900	9,000
Debt securities, gross unrealized losses		(5,200)	(2,300)
Debt securities, fair value		525,100	551,700
Debt Securities | Mortgage and asset backed securities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[2]	1,653,700	1,592,100
Debt securities, gross unrealized gains	[2]	9,200	13,800
Debt securities, gross unrealized losses	[2]	(3,400)	(1,300)
Debt securities, fair value	[2]	1,659,500	1,604,600
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Equity securities, cost			3,165,800
Equity securities, gross unrealized gains			932,500
Equity securities, gross unrealized losses			(3,800)
Equity securities, fair value		4,540,700	4,094,500
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		5,200	5,000
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		611,700	578,100
Fair Value		$ 611,700	$ 578,100

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

[2] Includes $1,071.8 million and $1,101.3 million of collateralized loan obligations as of March 31, 2018 and December 31, 2017, respectively.