Debt - Narrative (Details) - USD ($)			12 Months Ended
	Dec. 07, 2017	Oct. 10, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
FHLB borrowings	$ 25,000,000	$ 50,000,000	$ 50,000,000	$ 0	$ 0
FHLB advances interest rate			1.57%
Current Unsecured Bank Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 150,000,000
Basis spread on variable rate			1.15%
Variable commitment fee			0.15%
4.50% Senior Notes
Debt Instrument [Line Items]
Stated rate			4.50%
Principal amount			$ 250,000,000
Discount percent on issuance			0.265%
Effective interest percentage			4.533%
October 5, 2022
Debt Instrument [Line Items]
FHLB Advances			$ 25,000,000
December 2, 2022
Debt Instrument [Line Items]
FHLB Advances			25,000,000
Long-term debt
Debt Instrument [Line Items]
FHLB Advances			$ 50,000,000