Fair Value Measurements for Available for Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of available-for-sale securities	$ 6,500,541	$ 6,738,618
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of available-for-sale securities	2,266,877	1,286,625
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of available-for-sale securities	4,233,664	5,451,993
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of available-for-sale securities	$ 0	$ 0