Summary of Significant Accounting Policies (Effect of New Financial Accounting Standards) (Details) - USD ($) $ in Thousands	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets		$ 23,200
Present value of lease liabilities		24,617
Net deferred tax asset		13,600		$ 19,499
Adjustment to retained earnings		702,502		579,252
Increase in allowance		70,395		61,963	$ 55,686	$ 54,324
Reclassification to debt securities available for sale		3,294,361
Reclassification from held to maturity securities		$ (91,324)		$ (236,283)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets			$ 25,900
Present value of lease liabilities			27,600
Net deferred tax asset			440
Adjustment to retained earnings			1,300
Accounting Standards Update 2016-13 | Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross loans established under purchase accounting valuations	$ 1,800,000
Accounting Standards Update 2016-13 | Forecast | Unfunded Loan Commitment | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in allowance	17,600
Accounting Standards Update 2016-13 | Forecast | Unfunded Loan Commitment | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in allowance	$ 28,200
Accounting Standards Update 2017-12
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification to debt securities available for sale			148,000
Reclassification from held to maturity securities			$ 148,000