Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Dec. 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		$ 578,054	$ 778,410
Mortgage and asset-backed securities, amortized cost or cost	[1]	3,131,000
Debt securities with maturity dates, amortized cost or cost:
One year or less		415,500
Over one through five years		3,039,400
Over five through ten years		3,228,000
Over ten years		2,722,900
Debt securities, amortized cost		12,536,772	12,927,103
Equity securities, cost		3,170,673	2,816,572
Amortized Cost or Cost		16,285,600	16,522,100
Short-term investments due in one year or less, fair value		578,100
Mortgage and asset-backed securities, fair value	[1]	3,151,900
Debt securities with maturity dates, fair value:
One year or less		416,400
Over one through five years		3,055,800
Over five through ten years		3,270,400
Over ten years		2,826,900
Total debt securities, fair value		12,721,399	12,983,213
Equity securities, fair value		4,099,467	3,109,523
Fair Value		$ 17,399,000	$ 16,871,100

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