Fair Value Measurements (Details 1) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Securities available‑for‑sale	$ 1,809,364	$ 1,567,177
Level 1
Securities available‑for‑sale	522	519
Level 2
Securities available‑for‑sale	1,777,816	1,532,711
Level 3
Securities available‑for‑sale	31,026	33,947
Government agency and government-sponsored enterprise pass through securities [Member]
Securities available‑for‑sale	407,598	535,672
Government agency and government-sponsored enterprise collateralized mortgage obligations [Member]
Securities available‑for‑sale	211,419	277,946
Covered private label collateralized mortgage obligation [Member]
Securities available‑for‑sale	31,026	33,947
Other Private Label Collateralized Mortgage Obligations [Member]
Securities available‑for‑sale	7,262	10,914
Municipal securities
Securities available‑for‑sale	858,837	536,116
Corporate Debt Securities [Member]
Securities available‑for‑sale	47,396	110,109
Collateralized Loan Obligations [Member]
Securities available‑for‑sale	132,317	0
US Government-sponsored Enterprises Debt Securities [Member]
Securities available‑for‑sale	37,524	36,757
Other securities [Member]
Securities available‑for‑sale	24,437	25,716
Fair Value, Measurements, Recurring [Member]
Securities available‑for‑sale	1,809,364	1,567,177
Derivative Asset, Fair Value of Collateral	10,576	5,234
Assets, Fair Value Disclosure, Recurring	1,819,940	1,572,411
Derivative Liability, Fair Value of Collateral	526	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	522	519
Derivative Liability, Fair Value of Collateral	0	0
Fair Value, Measurements, Recurring [Member] | Level 2
Securities available‑for‑sale		1,532,711
Derivative Asset, Fair Value of Collateral	10,576	5,234
Assets, Fair Value Disclosure, Recurring	1,788,392	1,537,945
Derivative Liability, Fair Value of Collateral	526	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	31,026	33,947
Derivative Liability, Fair Value of Collateral	0	0
Fair Value, Measurements, Recurring [Member] | Government agency and government-sponsored enterprise pass through securities [Member]
Securities available‑for‑sale		535,672
Fair Value, Measurements, Recurring [Member] | Government agency and government-sponsored enterprise pass through securities [Member] | Level 1
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Government agency and government-sponsored enterprise pass through securities [Member] | Level 2
Securities available‑for‑sale	407,598	535,672
Fair Value, Measurements, Recurring [Member] | Government agency and government-sponsored enterprise pass through securities [Member] | 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		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	211,419	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		33,947
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	31,026	33,947
Fair Value, Measurements, Recurring [Member] | Other Private Label Collateralized Mortgage Obligations [Member]
Securities available‑for‑sale		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	7,262	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		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	858,837	536,116
Fair Value, Measurements, Recurring [Member] | Municipal securities | Level 3
Securities available‑for‑sale	0	0
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member] | Level 1
Securities available‑for‑sale	0
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member] | Level 2
Securities available‑for‑sale	51,548
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member] | Level 3
Securities available‑for‑sale	0
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]
Securities available‑for‑sale		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	47,396	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] | Level 1
Securities available‑for‑sale	0
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 2
Securities available‑for‑sale	132,317
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 3
Securities available‑for‑sale	0
Fair Value, Measurements, Recurring [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Securities available‑for‑sale		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	37,524	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		25,716
Fair Value, Measurements, Recurring [Member] | Other securities [Member] | Level 1
Securities available‑for‑sale	522	519
Fair Value, Measurements, Recurring [Member] | Other securities [Member] | Level 2
Securities available‑for‑sale	23,915	25,197
Fair Value, Measurements, Recurring [Member] | Other securities [Member] | Level 3
Securities available‑for‑sale	$ 0	$ 0