Available-for-Sale Securities - Additional Information (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($) Investment	Dec. 31, 2018 USD ($) Investment
Schedule Of Available For Sale Securities [Line Items]
Weighted average contractual maturity, Terms	16 years 3 months 18 days
CRE CLO Investments
Schedule Of Available For Sale Securities [Line Items]
Number of investments purchased \| Investment	10
Weighted average coupon rate	2.20%
Aggregate purchase price of investments \| $	$ 234,200,000
CMBS and CRE CLO Investments
Schedule Of Available For Sale Securities [Line Items]
Number of Investments \| Investment	14	4
Other than temporary impairments on available-for-sale \| $	$ 0	$ 0

Amount of cash outflow to acquire investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3213-108585

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3151-108585

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26853-111562

Name:	us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration