Postretirement Benefit Plans - Activity for the SERP (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Premiums paid for insurance coverage	$ 1,047	$ 2,050
Liability	594,354	602,605
SERPs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total investments	95,206	95,715
SERPs [Member] | Nonqualified plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	84,828	84,465
SERP (Active) [Member] | Nonqualified plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	82,200	81,457
SERP (Closed) [Member] | Nonqualified plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	2,628	3,008
Company owned life insurance [Member] | SERPs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total investments	41,608	40,273
Exchange traded fund [Member] | SERPs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total investments	$ 53,598	$ 55,442