Investment Securities - Summary of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Detail) $ in Thousands	Jun. 30, 2015 USD ($)
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 177,307
Due after one year through five years, Amortized Cost	2,063,160
Due after five years through ten years, Amortized Cost	659,129
Due after ten years, Amortized Cost	213,743
Total Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	3,113,339
Due in one year or less, Fair Value	180,332
Due after one year through five years, Fair Value	2,104,524
Due after five years through ten years, Fair Value	652,414
Due after ten years, Fair Value	216,947
Total, Available-for-sale, Fair Value	$ 3,154,217