Fair Value Measurements (Details 1) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Securities available‑for‑sale	$ 1,595,409	$ 1,567,177
Derivative Asset, Fair Value of Collateral	9,749	5,234
Derivative Liability, Fair Value of Collateral		118
Level 1
Securities available‑for‑sale	524	519
Derivative Asset, Fair Value of Collateral	0	0
Derivative Liability, Fair Value of Collateral		0
Level 2
Securities available‑for‑sale	1,561,441	1,532,711
Derivative Asset, Fair Value of Collateral	9,749	5,234
Derivative Liability, Fair Value of Collateral		118
Level 3
Securities available‑for‑sale	33,444	33,947
Derivative Asset, Fair Value of Collateral	0	0
Derivative Liability, Fair Value of Collateral		0
Collateralized Loan Obligations [Member]
Securities available‑for‑sale		0
Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	1,595,409	1,567,177
Derivative Asset, Fair Value of Collateral	9,749	5,234
Assets, Fair Value Disclosure, Recurring	1,605,158	1,572,411
Derivative Liability, Fair Value of Collateral		118
Fair Value, Measurements, Recurring [Member] | Level 1
Securities available‑for‑sale		519
Derivative Asset, Fair Value of Collateral	0	0
Assets, Fair Value Disclosure, Recurring	524	519
Derivative Liability, Fair Value of Collateral		0
Fair Value, Measurements, Recurring [Member] | Level 2
Securities available‑for‑sale		1,532,711
Derivative Asset, Fair Value of Collateral	9,749	5,234
Assets, Fair Value Disclosure, Recurring	1,571,190	1,537,945
Derivative Liability, Fair Value of Collateral		118
Fair Value, Measurements, Recurring [Member] | Level 3
Securities available‑for‑sale		33,947
Derivative Asset, Fair Value of Collateral	0	0
Assets, Fair Value Disclosure, Recurring	33,444	33,947
Derivative Liability, Fair Value of Collateral		0
Fair Value, Measurements, Recurring [Member] | Government agency and government-sponsored enterprise pass through securities
Securities available‑for‑sale	458,892	535,672
Fair Value, Measurements, Recurring [Member] | Government agency and government-sponsored enterprise pass through securities | Level 1
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Government agency and government-sponsored enterprise pass through securities | Level 2
Securities available‑for‑sale	458,892	535,672
Fair Value, Measurements, Recurring [Member] | Government agency and government-sponsored enterprise pass through securities | Level 3
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Government agency and government-sponsored enterprise collateralized mortgage obligations [Member]
Securities available‑for‑sale	276,224	277,946
Fair Value, Measurements, Recurring [Member] | Government agency and government-sponsored enterprise collateralized mortgage obligations [Member] | Level 1
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Government agency and government-sponsored enterprise collateralized mortgage obligations [Member] | Level 2
Securities available‑for‑sale	276,224	277,946
Fair Value, Measurements, Recurring [Member] | Government agency and government-sponsored enterprise collateralized mortgage obligations [Member] | Level 3
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Covered private label collateralized mortgage obligation [Member]
Securities available‑for‑sale	33,444	33,947
Fair Value, Measurements, Recurring [Member] | Covered private label collateralized mortgage obligation [Member] | Level 1
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Covered private label collateralized mortgage obligation [Member] | Level 2
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Covered private label collateralized mortgage obligation [Member] | Level 3
Securities available‑for‑sale	33,444	33,947
Fair Value, Measurements, Recurring [Member] | Other Private Label Collateralized Mortgage Obligations [Member]
Securities available‑for‑sale	9,650	10,914
Fair Value, Measurements, Recurring [Member] | Other Private Label Collateralized Mortgage Obligations [Member] | Level 1
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Other Private Label Collateralized Mortgage Obligations [Member] | Level 2
Securities available‑for‑sale	9,650	10,914
Fair Value, Measurements, Recurring [Member] | Other Private Label Collateralized Mortgage Obligations [Member] | Level 3
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Municipal securities
Securities available‑for‑sale	592,853	536,116
Fair Value, Measurements, Recurring [Member] | Municipal securities | Level 1
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Municipal securities | Level 2
Securities available‑for‑sale	592,853	536,116
Fair Value, Measurements, Recurring [Member] | Municipal securities | Level 3
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]
Securities available‑for‑sale	47,745	110,109
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 1
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 2
Securities available‑for‑sale	47,745	110,109
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 3
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member]
Securities available‑for‑sale	113,805
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 2
Securities available‑for‑sale	113,805
Fair Value, Measurements, Recurring [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Securities available‑for‑sale	37,405	36,757
Fair Value, Measurements, Recurring [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Level 1
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Level 2
Securities available‑for‑sale	37,405	36,757
Fair Value, Measurements, Recurring [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Level 3
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Other securities [Member]
Securities available‑for‑sale	25,391	25,716
Fair Value, Measurements, Recurring [Member] | Other securities [Member] | Level 1
Securities available‑for‑sale	524	519
Fair Value, Measurements, Recurring [Member] | Other securities [Member] | Level 2
Securities available‑for‑sale	24,867	25,197
Fair Value, Measurements, Recurring [Member] | Other securities [Member] | Level 3
Securities available‑for‑sale	$ 0	$ 0