Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Sep. 30, 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		$ 547,787	$ 778,410
Mortgage and asset-backed securities, amortized cost or cost	[1]	3,124,200
Debt securities with maturity dates, amortized cost or cost:
One year or less		385,200
Over one through five years		3,089,600
Over five through ten years		3,359,400
Over ten years		3,069,200
Debt securities, amortized cost		13,027,581	12,927,103
Equity securities, cost		3,174,837	2,816,572
Amortized Cost or Cost		16,750,200	16,522,100
Short-term investments due in one year or less, fair value		547,800
Mortgage and asset-backed securities, fair value	[1]	3,143,900
Debt securities with maturity dates, fair value:
One year or less		386,100
Over one through five years		3,124,600
Over five through ten years		3,414,900
Over ten years		3,142,900
Total debt securities, fair value		13,212,395	12,983,213
Equity securities, fair value		3,821,601	3,109,523
Fair Value		$ 17,581,800	$ 16,871,100

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