Benefit Plans - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Minimum percentages of gain loss consider for amortization (in hundredths)	10.00%
Minimum value of outstanding noncallable bonds used in hypothetical cash flow bond matching exercise	$ 50,000,000
Future earnings period used in determining the expected average rate of earnings	20 years
Minimum percentage return should exceed growth in consumer price index annually (in hundredths)	5.75%
Maximum investment in international mutual funds (in hundredths)	30.00%
Percent of international mutual funds equity allocation in emerging markets (in hundredths)	3.00%
Percent of international mutual funds equity allocation in companies primarily based in Europe and the Pacific Basin (in hundredths)	21.00%
Discretionary profit sharing contribution as a percentage of participant's eligible compensation (in hundredths)	5.00%
Matching contribution rate on employees' before-tax contributions, first contribution level (in hundredths)	80.00%
Employee contributions subject to employer match, first contribution level	$ 1,000
Matching contribution rate on employees' before-tax contributions, second contribution level (in hundredths)	40.00%
Employee contributions subject to employer match, second contribution level	$ 2,000
Matching contribution rate on employees' before-tax contributions, hired after 01/01/2014 (in hundredths)	100.00%
Employee contributions subject to employer match for employees hired after 01/01/2014 (in hundredths)	4.00%
Profit sharing and 401(k) savings plan expenses	$ 6,000,000	$ 5,900,000	$ 5,100,000
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Lump sum pension plan benefit payments	$ 6,300,000	$ 11,200,000
Pension Plan | Equity securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations (in hundredths)	40.00%
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Health care trend rate used for current year (in hundredths)	6.50%
Health Care Cost Trend Rate Assumed for Next Year (in hundredths)	6.50%	6.50%
Rate to Which the Cost Trend Rate is Assumed to Decline (Ultimate Trend Rate) (in hundredths)	5.00%	5.00%
Other Postretirement Benefit Plan | Equity securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations (in hundredths)	100.00%