Retirement Plans - Narrative (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Additional cash contribution to TRP for the remainder of the year	$ 0			$ 0
Pension lump-sum payment charge	1,200,000	$ 6,300,000
Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net pension plan obligation	130,900,000			130,900,000
Contributions to plan				11,100,000	$ 10,900,000
Pension lump-sum payment charge	1,198,000		$ 0	7,498,000	0
SERP
Defined Benefit Plan Disclosure [Line Items]
Contributions to plan				32,900,000	$ 7,200,000
Accrued Liabilities | Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net pension plan obligation	8,400,000			8,400,000
KFMB
Defined Benefit Plan Disclosure [Line Items]
Net pension plan obligation	$ 7,300,000			$ 7,300,000