Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Short-term investments due in one year or less, amortized cost or cost		$ 1,725,088	$ 778,410
Mortgage and asset-backed securities, amortized cost or cost	[1]	3,318,400
Debt securities with maturity dates, amortized cost or cost:
One year or less		499,100
Over one through five years		2,971,400
Over five through ten years		3,195,200
Over ten years		2,980,200
Debt securities, amortized cost		12,964,344	12,927,103
Equity securities, cost		1,816,796	2,816,572
Amortized Cost or Cost		16,506,200	16,522,100
Short-term investments due in one year or less, fair value		1,725,100
Mortgage and asset-backed securities, fair value	[1]	3,323,600
Debt securities with maturity dates, fair value:
One year or less		501,100
Over one through five years		3,002,500
Over five through ten years		3,228,000
Over ten years		2,997,300
Total debt securities, fair value		13,052,529	12,983,213
Equity securities, fair value		2,215,477	3,109,523
Fair Value		$ 16,993,100	$ 16,871,100

[1] Mortgage and asset-backed securities by their nature do not generally have single maturity dates.