Fair Value Measurements - Assets measured on nonrecurring basis (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities held to maturity, estimated fair value		$ 1,967,996		$ 1,040,543
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities held to maturity, estimated fair value		1,033,648		$ 1,040,543
Fair Value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities held to maturity, estimated fair value		2,260	$ 2,013
Investment securities held to maturity recorded as other than temporary impairment losses		(309)	(242)
Impaired loans, fair value	[1]	7,251	3,510
Impaired loans, total gains (losses)	[1]	(4,559)	(15,921)
OREO, fair value		16,147	25,113
OREO, total gains (losses)		(527)	(1,723)
Fair Value, measurements, nonrecurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities held to maturity, estimated fair value			2,013
Impaired loans, fair value	[1]	7,251	3,510
OREO, fair value		$ 16,147	$ 25,113

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