Short-Term Borrowings and Long-Term Debt Subordinated Debt (Details)	1 Months Ended	3 Months Ended	12 Months Ended
	Jun. 30, 2015	Sep. 30, 2015 USD ($)	Dec. 31, 2016 USD ($) subsidiarytrusts	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2016	Nov. 30, 2014 USD ($)	May 31, 2007 USD ($)
Subordinated Debt [Abstract]
Net Income			$ 161,625,000	$ 149,502,000	$ 157,894,000
Income taxes			$ 46,624,000	$ 49,921,000	$ 52,606,000
Subsidiary trusts owned by parent | subsidiarytrusts			3
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
Non-Interest Income (Expense) | Junior subordinated deferrable interest debentures
Subordinated Debt [Abstract]
Loss on debt extinguishment		(5,600,000)
Unrealized Effective Portions of Losses on Forward-Starting Interest Rate Swaps | Reclassification out of Accumulated Other Comprehensive Income | Junior subordinated deferrable interest debentures
Subordinated Debt [Abstract]
Net Income		2,500,000
Income taxes		1,300,000
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			3.49%
Subordinated Debt			$ 6,186,000
Maturity			Jun. 30, 2034
Callable			Mar. 31, 2017
Call Price			1.000
Junior subordinated deferrable interest debentures | Columbia Bancorp Statutory Trust II
Subordinated Debt [Abstract]
Interest Rate			2.85%
Subordinated Debt			$ 4,124,000
Maturity			Mar. 15, 2035
Callable			Mar. 31, 2017
Call Price			1.000
Junior subordinated deferrable interest debentures | Columbia Bancorp Statutory Trust III
Subordinated Debt [Abstract]
Interest Rate			2.73%
Subordinated Debt			$ 6,186,000
Maturity			Jun. 15, 2035
Callable			Mar. 31, 2017
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]
Effective interest rate								5.96%
Subordinated Debt								$ 100,000,000
Stated interest rate								5.75%
Subordinated debt | November 2024 Subordinated Debt
Subordinated Debt [Abstract]
Effective interest rate							4.87%
Subordinated Debt							$ 100,000,000
Stated interest rate							4.50%