Summary of Available-For-Sale Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Amortized Cost
Less than one year	$ 849,288
Greater than one year but less than two years	253,091
Amortized Cost	1,102,379	$ 807,843
Fair Value
Less than one year	848,217
Greater than one year but less than two years	251,839
Total	$ 1,100,056	$ 806,750