Summary of Significant Accounting Policies (Effect of New Financial Accounting Standards) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from accumulated other comprehensive income (loss) to retained earnings	$ 0
Reclassification to debt securities available for sale			$ 2,433,623,000
Reclassification from held to maturity securities	(253,550,000)		$ (236,283,000)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from accumulated other comprehensive income (loss) to retained earnings	280,000
Retained Earnings | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from accumulated other comprehensive income (loss) to retained earnings				$ 280,000
Retained Earnings | Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Tax Act, cumulative-effect adjustment	4,500,000
AOCI
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from accumulated other comprehensive income (loss) to retained earnings	(280,000)
AOCI | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from accumulated other comprehensive income (loss) to retained earnings				$ (280,000)
AOCI | Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Tax Act, cumulative-effect adjustment	$ (4,500,000)
Subsequent Event | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset		$ 24,200,000
Operating lease, liability		26,000,000
Subsequent Event | Accounting Standards Update 2017-12
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification to debt securities available for sale		148,400,000
Reclassification from held to maturity securities		$ 148,400,000