Basis of Presentation - Adoption of New Accounting Standards (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses: loans	$ 512,958	$ 589,828		$ 304,924	$ 304,410	$ 301,888	$ 280,903
Allowance for credit losses: held-to-maturity securities	291	222		0
Deferred tax assets				28,433
Retained earnings, net of tax	5,283,433			4,575,601
Unfunded Credit Commitments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses: loans	$ 101,515	99,294		67,656	$ 63,108	62,664	55,183
Adjustments Due to Adoption of ASC 326
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses: loans				25,464
Allowance for credit losses: held-to-maturity securities				174
Deferred tax assets				13,415
Retained earnings, net of tax				(35,049)
Adjustments Due to Adoption of ASC 326 | Unfunded Credit Commitments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses: loans		$ 0		$ 22,826		$ 0	$ 0
Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses: loans			$ 330,388
Allowance for credit losses: held-to-maturity securities			174
Deferred tax assets			41,848
Retained earnings, net of tax			4,540,552
Adjusted Balance | Unfunded Credit Commitments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses: loans			$ 90,482