Amortized Cost and Fair Value of Available for Sale Securities by Expected Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 1,256,912	$ 1,592,144
Due after one year through five years	2,583,254	3,218,371
Due after five years through ten years	389,425	680,641
Due after ten years	10,127	14,096
Total	4,239,718	5,505,252
Amortized cost of available-for-sale securities	6,376,091	6,691,140
Fair Value
Due in one year or less	1,256,724	1,591,740
Due after one year through five years	2,577,163	3,187,739
Due after five years through ten years	387,386	656,162
Due after ten years	10,551	14,629
Total	4,231,824	5,450,270
Fair value of available-for-sale securities	6,500,541	6,738,618
Restricted investments
Amortized Cost
Amortized cost of available-for-sale securities	164,548	164,548
Fair Value
Fair value of available-for-sale securities	165,011	164,085
Equity securities
Amortized Cost
Amortized cost of available-for-sale securities	1,971,825	1,021,340
Fair Value
Fair value of available-for-sale securities	$ 2,103,706	$ 1,124,263