Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets
Securities available for sale	$ 2,827,805	$ 2,158,853
Mortgage loans held for sale	202,168	139,950
Derivative instruments	30,086	28,798
Liabilities
Derivative instruments	24,534	15,411
Level 1
Assets
Derivative instruments	0	0
Liabilities
Derivative instruments	0	0
Level 2
Assets
Derivative instruments	41,440	32,903
Liabilities
Derivative instruments	38,147	31,354
Level 3
Assets
Derivative instruments	0	0
Liabilities
Derivative instruments	0	0
Recurring
Assets
Securities available for sale	2,827,805	2,158,853
Mortgage loans held for sale	202,168	139,950
Derivative instruments	41,440	32,903
Total	3,071,413	2,331,706
Liabilities
Derivative instruments	38,147	31,354
Recurring | Level 1
Assets
Securities available for sale	0	0
Mortgage loans held for sale	0	0
Derivative instruments	0	0
Total	0	0
Liabilities
Derivative instruments	0	0
Recurring | Level 2
Assets
Securities available for sale	2,827,805	2,158,853
Mortgage loans held for sale	202,168	139,950
Derivative instruments	41,440	32,903
Total	3,071,413	2,331,706
Liabilities
Derivative instruments	38,147	31,354
Recurring | Level 3
Assets
Securities available for sale	0	0
Mortgage loans held for sale	0	0
Derivative instruments	0	0
Total	0	0
Liabilities
Derivative instruments	$ 0	$ 0