Fair Value of Financial Instruments - Assets measured on nonrecurring basis (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity, estimated fair value		$ 1,142,694		$ 1,182,009
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		1,142,694		$ 1,182,009
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		2,013
Investment securities held to maturity recorded as other than temporary impairment losses		(242)
Impaired loans, fair value	[1]	3,510	$ 69,032
Impaired loans, total gains (losses)	[1]	15,921	(15,408)
OREO, fair value		25,113	17,877
OREO, total gains (losses)		(1,723)	(699)
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		2,013
Impaired loans, fair value	[1]	3,510	69,032
OREO, fair value		$ 25,113	$ 17,877

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