Amortized Cost or Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		$ 12,789,700	$ 16,285,600
Gross Unrealized Gains		98,900	1,170,500
Gross Unrealized Losses		(170,800)	(57,100)
Fair Value		12,717,800	17,399,000
Equity securities, cost			3,170,673
Equity securities, gross unrealized gains			932,600
Equity securities, gross unrealized losses			(3,800)
Equity securities, fair value			4,099,467
Debt securities, amortized cost		11,895,850	12,536,772
Debt securities, gross unrealized gains		98,900	237,900
Debt securities, gross unrealized losses		(170,800)	(53,300)
Debt securities, fair value		11,823,968	12,721,399
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,042,400	963,900
Debt securities, gross unrealized gains		2,400	1,700
Debt securities, gross unrealized losses		(22,400)	(17,600)
Debt securities, fair value		1,022,400	948,000
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,177,500	3,578,900
Debt securities, gross unrealized gains		44,400	109,800
Debt securities, gross unrealized losses		(7,200)	(6,600)
Debt securities, fair value		2,214,700	3,682,100
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		939,000	1,000,100
Debt securities, gross unrealized gains		12,300	11,200
Debt securities, gross unrealized losses		(3,400)	(4,700)
Debt securities, fair value		947,900	1,006,600
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,431,400	2,381,100
Debt securities, gross unrealized gains		13,200	61,600
Debt securities, gross unrealized losses		(59,300)	(9,700)
Debt securities, fair value		2,385,300	2,433,000
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,363,000	1,481,800
Debt securities, gross unrealized gains		9,100	24,500
Debt securities, gross unrealized losses		(18,800)	(6,500)
Debt securities, fair value		1,353,300	1,499,800
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,392,400	993,900
Debt securities, gross unrealized gains		10,300	6,300
Debt securities, gross unrealized losses		(14,800)	(4,600)
Debt securities, fair value	[1]	1,387,900	995,600
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		536,900	545,000
Debt securities, gross unrealized gains		2,800	9,000
Debt securities, gross unrealized losses		(6,400)	(2,300)
Debt securities, fair value		533,300	551,700
Other Asset Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[2]	2,013,300	1,592,100
Debt securities, gross unrealized gains	[2]	4,400	13,800
Debt securities, gross unrealized losses	[2]	(38,500)	(1,300)
Debt securities, fair value	[2]	1,979,200	1,604,600
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			4,094,500
Preferred Stock
Schedule of Available-for-sale Securities [Line Items]
Equity securities, cost			4,900
Equity securities, gross unrealized gains			100
Equity securities, gross unrealized losses			0
Equity securities, fair value			5,000
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		893,800	578,100
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Fair Value		$ 893,800	$ 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 $1,266.9 million and $1,101.3 million of collateralized loan obligations as of December 31, 2018 and 2017, respectively.