Subsequent Events - CMBS Sale (Details) $ in Thousands		9 Months Ended
	Oct. 02, 2018 USD ($) security	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 security
Subsequent Event [Line Items]
Proceeds from sale of investments and other assets | $		$ 10,880	$ 0
Commercial Mortgage Backed Securities [Member]
Subsequent Event [Line Items]
Number of securities sold | security		3
Number of securities held | security		3		8
Proceeds from sale of investments and other assets | $		$ 9,900
Commercial Mortgage Backed Securities [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of securities sold | security	2
Proceeds from sale of investments and other assets | $	$ 12,800