Basis of Presentation and Significant Accounting Policies - Measurement of Credit Losses on Financial Instruments (Details) $ / shares in Units, loan in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) loan	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($) $ / shares	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of ASU 2016-13	$ (934,462)	$ (964,336)	$ (961,059)		$ (1,019,136)	$ (1,023,240)	$ (1,027,736)	$ (981,131)	$ (827,531)
Threshold period when delinquent loans are placed on nonaccrual status	90 days
Number of commercial real estate loans included in third-party historical loan loss database | loan	100
Loans and securities					4,257,086
Allowance for credit losses					0
Loans and securities, net					4,257,086
Liability for off balance sheet credit losses	$ 7,400				0
Cumulative earnings	$ 80,014				162,076
Cumulative effect of adoption of new accounting principle
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of ASU 2016-13				$ (18,500)	$ 18,472				0
Cumulative effect of adoption of ASU 2016-13 per share (in usd per share) | $ / shares					$ 0.34
Loans and securities					$ 0
Allowance for credit losses					(16,692)
Loans and securities, net					(16,692)
Liability for off balance sheet credit losses					1,780
Cumulative earnings					(18,472)
Stockholders' equity, adjusted balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of ASU 2016-13					(1,000,664)				$ (827,531)
Loans and securities					4,257,086
Allowance for credit losses					(16,692)
Loans and securities, net					4,240,394
Liability for off balance sheet credit losses					1,780
Cumulative earnings					$ 143,604