Investments (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 1,547,015	$ 1,522,871
Available-for-sale Securities, Gross Unrealized Gain	51,805	47,908
Available-for-sale Securities, Gross Unrealized Loss	(3,411)	(3,602)
Securities available‑for‑sale	1,595,409	1,567,177
Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available‑for‑sale	1,595,409	1,567,177
Government agency and government-sponsored enterprise pass through securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	439,696	515,902
Available-for-sale Securities, Gross Unrealized Gain	19,327	20,142
Available-for-sale Securities, Gross Unrealized Loss	(131)	(372)
Government agency and government-sponsored enterprise pass through securities | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available‑for‑sale	458,892	535,672
Government agency and government-sponsored enterprise collateralized mortgage obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	269,766	275,513
Available-for-sale Securities, Gross Unrealized Gain	6,909	3,513
Available-for-sale Securities, Gross Unrealized Loss	(451)	(1,080)
Government agency and government-sponsored enterprise collateralized mortgage obligations [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available‑for‑sale	276,224	277,946
Covered private label collateralized mortgage obligation [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	26,496	26,889
Available-for-sale Securities, Gross Unrealized Gain	7,014	7,153
Available-for-sale Securities, Gross Unrealized Loss	(66)	(95)
Covered private label collateralized mortgage obligation [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available‑for‑sale	33,444	33,947
Other Private Label Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	9,645	10,961
Available-for-sale Securities, Gross Unrealized Gain	52	46
Available-for-sale Securities, Gross Unrealized Loss	(47)	(93)
Other Private Label Collateralized Mortgage Obligations [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available‑for‑sale	9,650	10,914
Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	579,344	521,499
Available-for-sale Securities, Gross Unrealized Gain	15,708	15,899
Available-for-sale Securities, Gross Unrealized Loss	(2,199)	(1,282)
Municipal securities | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available‑for‑sale	592,853	536,116
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	47,485	110,074
Available-for-sale Securities, Gross Unrealized Gain	683	597
Available-for-sale Securities, Gross Unrealized Loss	(423)	(562)
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available‑for‑sale	47,745	110,109
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	112,883	0
Available-for-sale Securities, Gross Unrealized Gain	922	0
Available-for-sale Securities, Gross Unrealized Loss	0	0
Securities available‑for‑sale		0
Collateralized Loan Obligations [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available‑for‑sale	113,805
US Government-sponsored Enterprises Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	36,249	36,232
Available-for-sale Securities, Gross Unrealized Gain	1,156	525
Available-for-sale Securities, Gross Unrealized Loss	0	0
US Government-sponsored Enterprises Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available‑for‑sale	37,405	36,757
Other securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	25,451	25,801
Available-for-sale Securities, Gross Unrealized Gain	34	33
Available-for-sale Securities, Gross Unrealized Loss	(94)	(118)
Other securities [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available‑for‑sale	$ 25,391	$ 25,716