Benefit Plans - Non-Qualified Retirement Plans (Details) - Nonqualified plan - Pension plan	12 Months Ended
	Dec. 31, 2017
Minimum
Non-Qualified Retirement Plans [Abstract]
Non-qualified retirement plan, number of years of highest average annual compensation used to determine benefits	5 years
Maximum
Non-Qualified Retirement Plans [Abstract]
Non-qualified retirement plan, number of years of highest average annual compensation used to determine benefits	10 years