Debt - Summary of Indebtedness (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Short-term debt:
Bank Credit Facility, expires June 21, 2024	$ 135,000,000	$ 0
Long-term debt:
Long-term debt	297,953,000	297,740,000
Debt and Lease Obligation	432,953,000	297,740,000
Senior Notes 4.50%
Long-term debt:
Long-term debt	247,953,000	247,740,000
Unamortized discount	$ 442,000	488,000
Stated rate	4.50%
Face amount	$ 250,000,000
Unamortized debt issuance costs	1,605,000	1,772,000
FHLB borrowing
Long-term debt:
Long-term debt	$ 50,000,000	$ 50,000,000