Fair Value Accounting and Measurement (Carrying Amounts and Estimated Fair Values of Selected Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Assets
Interest-earning deposits with banks	$ 736,422		$ 24,132
Debt securities available for sale at fair value	4,281,720		3,746,142
Equity Securities	13,425	$ 13,425	0	$ 0	$ 0	$ 0
Estimate of Fair Value Measurement [Member]
Assets
Cash and due from banks	193,823		223,541
Interest-earning deposits with banks	736,422		24,132
Debt securities available for sale at fair value	4,281,720		3,746,142
Equity Securities	13,425
FHLB stock	10,280		48,120
Loans held for sale	24,407		17,718
Loans	9,980,912		8,883,865
Interest rate contracts	53,202		19,144
Interest Rate Cash Flow Hedge Asset at Fair Value	35,668		14,727
Liabilities
Deposits	337,386		397,736
FHLB advances	9,354		952,762
Repurchase agreements	26,966		64,437
Subordinated Debentures	35,368		35,491
Interest rate contracts	53,677		19,145
Reported Value Measurement [Member]
Assets
Cash and due from banks	193,823		223,541
Interest-earning deposits with banks	736,422		24,132
Debt securities available for sale at fair value	4,281,720		3,746,142
Equity Securities	13,425
FHLB stock	10,280		48,120
Loans held for sale	24,407		17,718
Loans	9,531,979		8,659,497
Interest rate contracts	53,202		19,144
Interest Rate Cash Flow Hedge Asset at Fair Value	35,668		14,727
Liabilities
Deposits	337,228		400,070
FHLB advances	7,427		953,469
Repurchase agreements	26,966		64,437
Subordinated Debentures	35,139		35,277
Interest rate contracts	53,677		19,145
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value Measurement [Member]
Assets
Cash and due from banks	193,823		223,541
Interest-earning deposits with banks	736,422		24,132
Debt securities available for sale at fair value	0		0
Equity Securities	0
FHLB stock	0		0
Loans held for sale	0		0
Loans	0		0
Interest rate contracts	0		0
Interest Rate Cash Flow Hedge Asset at Fair Value	0		0
Liabilities
Deposits	0		0
FHLB advances	0		0
Repurchase agreements	0		0
Subordinated Debentures	0		0
Interest rate contracts	0		0
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value Measurement [Member]
Assets
Cash and due from banks	0		0
Interest-earning deposits with banks	0		0
Debt securities available for sale at fair value	4,281,720		3,746,142
Equity Securities	0
FHLB stock	10,280		48,120
Loans held for sale	24,407		17,718
Loans	0		0
Interest rate contracts	53,202		19,144
Interest Rate Cash Flow Hedge Asset at Fair Value	35,668		14,727
Liabilities
Deposits	337,386		397,736
FHLB advances	9,354		952,762
Repurchase agreements	26,966		64,437
Subordinated Debentures	35,368		35,491
Interest rate contracts	53,677		19,145
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value Measurement [Member]
Assets
Cash and due from banks	0		0
Interest-earning deposits with banks	0		0
Debt securities available for sale at fair value	0		0
Equity Securities	13,425
FHLB stock	0		0
Loans held for sale	0		0
Loans	9,980,912		8,883,865
Interest rate contracts	0		0
Interest Rate Cash Flow Hedge Asset at Fair Value	0		0
Liabilities
Deposits	0		0
FHLB advances	0		0
Repurchase agreements	0		0
Subordinated Debentures	0		0
Interest rate contracts	$ 0		$ 0