Fair Value - Quantitative Information About Nonrecurring Level 3 Fair Value Measurements (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 5,728,096	$ 5,661,847
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	7,055,335	6,930,762
Other real estate owned | Fair Value of Property | Fair Value on Nonrecurring Basis | Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 35,978	$ 34,645
Weighted Average	8.00%	7.86%
Impaired loans | Fair Value of Collateral | Fair Value on Nonrecurring Basis | Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 256	$ 254
Weighted Average	7.00%	7.29%