Summary of Available-For-Sale Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Less than one year	$ 1,173,776
Greater than one year but less than two years	648
Amortized Cost	1,174,424	$ 1,060,817
Fair Value
Less than one year	1,171,715
Greater than one year but less than two years	648
Total	$ 1,172,363	$ 1,059,176