Short-Term Borrowings and Long-Term Debt Subordinated Debt (Details)	1 Months Ended				3 Months Ended	12 Months Ended
	Jun. 30, 2015	Nov. 30, 2014 USD ($)	May. 31, 2007 USD ($)	Mar. 31, 2005 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) subsidiarytrusts	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Subordinated Debt [Abstract]
Subsidiary trusts owned by parent | subsidiarytrusts						3
Net Income						$ 149,502,000	$ 157,894,000	$ 161,840,000
Income tax benefit						49,921,000	$ 52,606,000	$ 51,085,000
Junior subordinated deferrable interest debentures
Subordinated Debt [Abstract]
Effective interest rate					6.52%
Extinguishment of debt, amount					$ 150,000,000
Amortization of financing costs					1,800,000
Junior subordinated deferrable interest debentures | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent | Reclassification out of Accumulated Other Comprehensive Income
Subordinated Debt [Abstract]
Net Income					2,500,000
Income tax benefit					1,300,000
Junior subordinated deferrable interest debentures | Non-Interest Income (Expense)
Subordinated Debt [Abstract]
Loss on debt extinguishment					5,600,000
Junior subordinated deferrable interest debentures
Subordinated Debt [Abstract]
Subordinated Debt						$ 16,496,000
Junior subordinated deferrable interest debentures | Columbia Bancorp Statutory Trust
Subordinated Debt [Abstract]
Interest Rate						2.88%
Subordinated Debt						$ 6,186,000
Maturity						Jun. 30, 2034
Callable						Mar. 31, 2016
Call Price						1.000
Junior subordinated deferrable interest debentures | Columbia Bancorp Statutory Trust II
Subordinated Debt [Abstract]
Interest Rate						2.40%
Subordinated Debt						$ 4,124,000
Maturity						Mar. 15, 2035
Callable						Mar. 15, 2016
Call Price						1.000
Junior subordinated deferrable interest debentures | Columbia Bancorp Statutory Trust III
Subordinated Debt [Abstract]
Interest Rate						2.28%
Subordinated Debt						$ 6,186,000
Maturity						Jun. 15, 2035
Callable						Mar. 15, 2016
Call Price						1.000
Subordinated debt
Subordinated Debt [Abstract]
Debt instrument, face amount					$ 150,000,000
Debt instrument, term	10 years
Interest Rate	4.50%
Effective interest rate					4.69%
Subordinated debt | May 2017 Subordinated Debt
Subordinated Debt [Abstract]
Debt instrument, term			10 years
Effective interest rate			5.96%
Subordinated Debt			$ 100,000,000
Stated interest rate			5.75%
Subordinated debt | April 2015 Subordinated Debt
Subordinated Debt [Abstract]
Debt instrument, term				10 years
Effective interest rate				5.49%
Subordinated Debt				$ 100,000,000
Subordinated debt | November 2024 Subordinated Debt
Subordinated Debt [Abstract]
Debt instrument, term		10 years
Effective interest rate		4.87%
Subordinated Debt		$ 100,000,000
Stated interest rate		4.50%