Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities, Assets	$ 2,342,613,000	$ 2,158,853,000
Mortgage loans held for sale, Assets	194,816,000	139,950,000
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, Assets	41,120,000	32,903,000
Derivative instruments, Liabilities	35,704,000	31,354,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities, Assets	2,342,613,000	2,158,853,000
Mortgage loans held for sale, Assets	194,816,000	139,950,000
Derivative instruments, Assets	41,120,000	32,903,000
Total, Assets	2,578,549,000	2,331,706,000
Derivative instruments, Liabilities	35,704,000	31,354,000
Total, Liabilities	35,704,000	31,354,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities, Assets	67,206,000
Total, Assets	67,206,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities, Assets	2,275,407,000	2,158,853,000
Mortgage loans held for sale, Assets	194,816,000	139,950,000
Derivative instruments, Assets	41,120,000	32,903,000
Total, Assets	2,511,343,000	2,331,706,000
Derivative instruments, Liabilities	35,704,000	31,354,000
Total, Liabilities	$ 35,704,000	$ 31,354,000