Fair Value of Financial Instruments - Assets measured on nonrecurring basis (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity, estimated fair value		$ 1,201,832		$ 1,244,121
OREO, fair value		17,877	$ 17,764
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity, estimated fair value		1,201,832		$ 1,244,121
Fair Value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity, estimated fair value			3,205
Investment securities held to maturity recorded as other than temporary impairment losses			(285)
Impaired loans, fair value	[1]	69,032	132,336
Impaired loans, total gains (losses)	[1]	(15,408)	(3,304)
OREO, total gains (losses)		(699)	(1,259)
Fair Value, measurements, nonrecurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity, estimated fair value			3,205
Impaired loans, fair value	[1]	69,032	132,336
OREO, fair value		$ 17,877	$ 17,764

[1] Total gains (losses) represent charge-offs on impaired loans for which adjustments are based on the appraised value of the collateral.