Loans Receivable - Schedule of fair values and risk ratings (Details) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2019 USD ($) loan	Dec. 31, 2018 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans	17
Fair Value \| $	$ 1,105.1	$ 913.0
Percentage	100.00%	100.00%
Number of loans collateralized by interest in a joint venture	1
BBB
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans	1
Fair Value \| $	$ 95.2
BBB \| Loans Receivable \| Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Percentage	8.60%
BB
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans	9
Fair Value \| $	$ 509.1
BB \| Loans Receivable \| Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Percentage	46.10%
B
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans	6
Fair Value \| $	$ 494.5
B \| Loans Receivable \| Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Percentage	44.70%
NR
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans	1
Fair Value \| $	$ 6.3
NR \| Loans Receivable \| Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Percentage	0.60%

Loans Receivable, Number Of Loans

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- Definition

Loans Receivable, Number Of Loans Collateralized By Interest In Joint Venture

+ References

No definition available.

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
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-URI http://asc.fasb.org/subtopic&trid=2196772

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