Fair Value of Financial Instruments, Part 3 (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Financial assets [Abstract]
Cash and cash equivalents	$ 698,072	$ 671,448	$ 630,655	$ 583,044
Securities available for sale	624,547	676,759
Held-to-maturity securities, carrying value	59,998	70,946
Held to maturity securities	63,896	75,342
Net loans, carrying value	3,236,856	3,112,005
Net loans	3,276,461	3,171,005
Accrued interest receivable	10,347	10,800
Financial liabilities [Abstract]
Demand deposits	354,162	331,425
Interest bearing deposits	3,742,711	3,700,828
Short-term borrowings	184,405	189,116
Accrued interest payable	508	548
Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	698,072	671,448
Securities available for sale	35	35
Held to maturity securities	0	0
Net loans	0	0
Accrued interest receivable	62	30
Financial liabilities [Abstract]
Demand deposits	354,162	331,425
Interest bearing deposits	2,573,723	2,537,583
Short-term borrowings	0	0
Accrued interest payable	73	100
Level 2 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	0
Securities available for sale	624,512	676,724
Held to maturity securities	63,896	75,342
Net loans	0	0
Accrued interest receivable	2,250	2,694
Financial liabilities [Abstract]
Demand deposits	0	0
Interest bearing deposits	1,168,988	1,163,245
Short-term borrowings	184,405	189,116
Accrued interest payable	435	448
Level 3 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	0
Securities available for sale	0	0
Held to maturity securities	0	0
Net loans	3,276,461	3,171,005
Accrued interest receivable	8,035	8,076
Financial liabilities [Abstract]
Demand deposits	0	0
Interest bearing deposits	0	0
Short-term borrowings	0	0
Accrued interest payable	0	0
Carrying Value [Member]
Financial assets [Abstract]
Cash and cash equivalents	698,072	671,448
Securities available for sale	624,547	676,759
Held-to-maturity securities, carrying value	59,998	70,946
Federal Reserve Bank and Federal Home Loan Bank stock	9,480	9,228
Net loans, carrying value	3,236,856	3,112,005
Accrued interest receivable	10,347	10,800
Financial liabilities [Abstract]
Demand deposits	354,162	331,425
Interest bearing deposits, carrying value	3,742,631	3,700,816
Short-term borrowings	184,405	189,116
Accrued interest payable	$ 508	$ 548