Summary of Significant Accounting Policies (Details)	1 Months Ended	12 Months Ended
	Aug. 31, 2015 $ / shares shares	Dec. 31, 2017 USD ($) class_business state	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 29, 2015
Subsidiary, Sale of Stock [Line Items]
Number of business classes | class_business		2
Number of states in which entity operates | state		50
Ownership percentage					50.00%
Unrecognized tax benefits		$ 0	$ 0
Income tax penalties and interest accrued		0	0
Unrealized investment gains (losses), net of tax		795,000	$ (2,139,000)	$ (274,000)
IPO
Subsidiary, Sale of Stock [Line Items]
Stock issued during period (in shares) | shares	3,300,000
IPO | Chief Executive Officer
Subsidiary, Sale of Stock [Line Items]
Stock issued during period (in shares) | shares	100,000
Price per share (in dollars per share) | $ / shares	$ 10.50
Equity securities
Subsidiary, Sale of Stock [Line Items]
Unrealized investment gains (losses), net of tax		$ 1,100,000