Recently Issued Accounting Pronouncements - Narrative (Details) - USD ($)		12 Months Ended
		Dec. 31, 2019		Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member
Net decrease to retained earnings balance		$ 2,012,594,000		$ 2,654,647,000	$ 2,002,917,000		$ 1,984,456,000	$ 2,007,064,000	$ 1,969,697,000
Allowance for credit losses on loans		35,698,000		282,271,000	115,422,000		35,026,000	37,856,000	36,072,000
Deferred income taxes, net		0		105,859,000
Loans held for investment		8,722,311,000		15,082,884,000
Allowance for credit losses on off-balance sheet credit exposures		3,300,000		22,000,000.0		$ 3,279,000
Allowance for credit losses on debt securities, Held-to-maturity						0
Allowance for credit losses on debt securities, Available-for-sale						0
Operating lease liabilities		46,498,000		96,057,000
Operating lease right of use assets		43,177,000		86,188,000
Commercial loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans						20,118,000
Loans held for investment		1,975,053,000		3,724,905,000
Accumulated Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net decrease to retained earnings balance		396,051,000		$ 247,078,000	$ 361,242,000		$ 343,366,000	$ 325,363,000	$ 300,407,000
Impact of CECL Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net decrease to retained earnings balance	[1]	(45,625,000)
Allowance for credit losses on loans		55,686,000				64,000,000.0
Deferred income taxes, net						18,300,000
Loans held for investment						55,700,000
Allowance for credit losses on off-balance sheet credit exposures						8,285,000
Allowance for credit losses on debt securities, Held-to-maturity						0
Allowance for credit losses on debt securities, Available-for-sale						0
Impact of CECL Adoption | Commercial loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans						9,519,000
Impact of CECL Adoption | Commercial loans | Investor loans secured by real estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans						16,100,000
Impact of CECL Adoption | Commercial loans | Business loans secured by real estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans						27,600,000
Impact of CECL Adoption | Commercial loans | Commercial loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans						9,500,000
Impact of CECL Adoption | Consumer loans | Retail loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans						2,500,000
Impact of CECL Adoption | Accumulated Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net decrease to retained earnings balance		$ (45,625,000)	[1]			$ (45,600,000)

[1] Related to the adoption of Accounting Standards Update 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. See Note 2 - Recently Issue Accounting Pronouncements for further discussion.