Investment Securities - Summary of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 156,040
Due after one year through five years, Amortized Cost	2,056,835
Due after five years through ten years, Amortized Cost	757,260
Due after ten years, Amortized Cost	113,447
Total Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	3,083,582
Due in one year or less, Fair Value	159,389
Due after one year through five years, Fair Value	2,107,877
Due after five years through ten years, Fair Value	754,160
Due after ten years, Fair Value	115,732
Total, Available-for-sale, Fair Value	$ 3,137,158