Postretirement Benefits - Activity for the SERP (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Premiums paid for insurance coverage	$ 2,997	$ 2,050
Liability	580,055	602,606
Nonqualified plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	85,732	84,465
SERP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total investments	100,752	95,715
SERP [Member] | Nonqualified plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	85,732	84,465
SERP (Active) [Member] | Nonqualified plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	83,320	81,457
SERP (Closed) [Member] | Nonqualified plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	2,412	3,008
Company owned life insurance [Member] | SERP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total investments	43,976	40,273
Exchange traded fund [Member] | SERP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total investments	$ 56,776	$ 55,442