Postretirement Benefits - Activity for the SERP (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 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	595,347	602,606
Nonqualified plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	85,382	84,465
SERPs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total investments	96,923	95,715
SERPs [Member] | Nonqualified plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	85,382	84,465
SERP (Active) [Member] | Nonqualified plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	82,862	81,457
SERP (Closed) [Member] | Nonqualified plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	2,520	3,008
Company owned life insurance [Member] | SERPs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total investments	41,872	40,273
Exchange traded fund [Member] | SERPs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total investments	$ 55,051	$ 55,442