Fair Value of Financial Instruments (Carrying Value of Commercial Mortgage Loans and Additional Information) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 9,896	$ 0
Total mortgage loans | Mortgage Loans by Region
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 9,896	$ 0
Percent of Total	100.00%	0.00%
Total mortgage loans | Mortgage Loans by Region | East North Central
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 3,245	$ 0
Percent of Total	32.80%	0.00%
Total mortgage loans | Mortgage Loans by Region | Southern Atlantic
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 6,651	$ 0
Percent of Total	67.20%	0.00%
Total mortgage loans | Mortgage Loans by Property Type
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 9,896	$ 0
Percent of Total	100.00%	0.00%
Total mortgage loans | Mortgage Loans by Property Type | Multifamily
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 3,245	$ 0
Percent of Total	32.80%	0.00%
Total mortgage loans | Mortgage Loans by Property Type | Office
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 6,651	$ 0
Percent of Total	67.20%	0.00%
Less than 65%
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 9,896	$ 0