Loans Held for Investment - Additional Information (Details)		3 Months Ended	9 Months Ended
	Jul. 16, 2018 USD ($)	Sep. 30, 2018 USD ($) Rating Loan	Sep. 30, 2018 USD ($) Rating Loan	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Rating Loan
Accounts Notes And Loans Receivable [Line Items]
Total loan commitment amount		$ 4,705,831,000	$ 4,705,831,000		$ 3,727,156,000
Unfunded loan commitments		527,102,000	527,102,000		529,040,000
Unaccreted discount		$ 0	0		$ 2,000,000
Sale of participation interest in non-core, fixed rate performing loan, cash consideration			$ 2,174,000	$ 65,054,000
Number of loans on non-accrual status | Loan		0	0		0
Reserve		$ 0	$ 0		$ 0
Weighted average risk rating | Rating		2.8	2.8		2.6
Moved from Two Risk Rating into One Risk Rating
Accounts Notes And Loans Receivable [Line Items]
Number of loans selected for risk rate changes | Loan		1
Moved from Two Risk Rating into Three Risk Rating
Accounts Notes And Loans Receivable [Line Items]
Number of loans selected for risk rate changes | Loan		1
Moved from Three Risk Rating into Two Risk Rating
Accounts Notes And Loans Receivable [Line Items]
Number of loans selected for risk rate changes | Loan		2
Non-core | Fixed Rate Loan Investment
Accounts Notes And Loans Receivable [Line Items]
Sale of participation interest in non-core, fixed rate performing loan, cash consideration	$ 2,700,000
Loss on sale of investments	$ 400,000
German American Capital Corporation
Accounts Notes And Loans Receivable [Line Items]
Number of loans originated or acquired | Loan			21
Total loan commitment amount		$ 1,900,000,000	$ 1,900,000,000
Unpaid principal balance		1,600,000,000	1,600,000,000
Unfunded loan commitments		$ 264,500,000	$ 264,500,000