Fair Value Measurements (Details 1) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Securities available‑for‑sale	$ 1,567,177	$ 1,494,745
Derivative Asset, Fair Value of Collateral	5,234
Derivative Liability, Fair Value of Collateral	118
Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	1,567,177	1,494,745
Derivative Asset, Fair Value of Collateral	5,234
Assets, Fair Value Disclosure, Recurring	1,572,411
Derivative Liability, Fair Value of Collateral	118
Level 1
Securities available‑for‑sale		507
Derivative Asset, Fair Value of Collateral	0
Derivative Liability, Fair Value of Collateral	0
Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	519	507
Derivative Asset, Fair Value of Collateral	0
Assets, Fair Value Disclosure, Recurring	519
Derivative Liability, Fair Value of Collateral	0
Level 2
Securities available‑for‑sale		1,456,334
Derivative Asset, Fair Value of Collateral	5,234
Derivative Liability, Fair Value of Collateral	118
Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	1,532,711	1,456,334
Derivative Asset, Fair Value of Collateral	5,234
Assets, Fair Value Disclosure, Recurring	1,537,945
Derivative Liability, Fair Value of Collateral	118
Level 3
Securities available‑for‑sale		37,904
Derivative Asset, Fair Value of Collateral	0
Derivative Liability, Fair Value of Collateral	0
Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	33,947	37,904
Derivative Asset, Fair Value of Collateral	0
Assets, Fair Value Disclosure, Recurring	33,947
Derivative Liability, Fair Value of Collateral	0
Government agency and government-sponsored enterprise pass through securities | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	535,672	707,188
Government agency and government-sponsored enterprise pass through securities | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Government agency and government-sponsored enterprise pass through securities | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	535,672	707,188
Government agency and government-sponsored enterprise pass through securities | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Government agency and government-sponsored enterprise collateralized mortgage obligations [Member] | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	277,946	192,873
Government agency and government-sponsored enterprise collateralized mortgage obligations [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Government agency and government-sponsored enterprise collateralized mortgage obligations [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	277,946	192,873
Government agency and government-sponsored enterprise collateralized mortgage obligations [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Covered private label collateralized mortgage obligation [Member] | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	33,947	37,904
Covered private label collateralized mortgage obligation [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Covered private label collateralized mortgage obligation [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Covered private label collateralized mortgage obligation [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	33,947	37,904
Other Private Label Collateralized Mortgage Obligations [Member]
Securities available‑for‑sale		0
Other Private Label Collateralized Mortgage Obligations [Member] | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	10,914
Other Private Label Collateralized Mortgage Obligations [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0
Other Private Label Collateralized Mortgage Obligations [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	10,914
Other Private Label Collateralized Mortgage Obligations [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0
Municipal securities | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	536,116	436,658
Municipal securities | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Municipal securities | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	536,116	436,658
Municipal securities | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	110,109	82,707
Corporate Debt Securities [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Corporate Debt Securities [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	110,109	82,707
Corporate Debt Securities [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
US Government-sponsored Enterprises Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	36,757	9,872
US Government-sponsored Enterprises Debt Securities [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
US Government-sponsored Enterprises Debt Securities [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	36,757	9,872
US Government-sponsored Enterprises Debt Securities [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	0	0
Other securities [Member] | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	25,716	27,543
Other securities [Member] | Level 1 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	519	507
Other securities [Member] | Level 2 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	25,197	27,036
Other securities [Member] | Level 3 | Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	$ 0	$ 0