Investments Cash, Cash Equivalent and Investments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Percent of high quality fixed maturity securities		75.50%
Fixed maturity securities		$ 1,231,039	$ 1,208,570
Fixed maturity securities percent		88.70%	89.30%
Equity securities		$ 15,068
Equity securities			$ 16,164
Equity securities percent		1.10%	1.20%
Mortgage loans		$ 186	$ 195
Mortgage loans percent		0.00%	0.00%
Policy loans		$ 80,825	$ 73,735
Policy loans percent		5.80%	5.50%
Real estate and other long-term investments		$ 7,223	$ 7,452
Real estate and other long-term investments percent		0.50%	0.60%
Short-term investments		$ 7,865	$ 0
Short-term investment percentage		0.60%	0.00%
Cash and cash equivalents		$ 45,492	$ 46,064	$ 35,510	$ 82,827
Cash and cash equivalents percent		3.30%	3.40%
Total cash, cash equivalents and investments		$ 1,387,698	$ 1,352,180
Total cash, cash equivalents and investments percent		100.00%	100.00%
Debt Securities, Held-to-maturity, Transfer, Amount	$ 209,700
Debt Securities, Held-to-maturity, Transfer, Unrealized Gain (Loss)	$ 500