Long-term Debt (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($) Rate	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Unamortized deferred loan costs | $	$ 21.0	$ 23.6
Long-term Debt | $	1,026.0	1,100.4
Contractual Quarterly Payments | $	$ 2.1
Leverage Ratio	2.40
Letters of Credit Outstanding, Amount | $	$ 3.9	$ 4.4
Maximum [Member]
Debt Instrument [Line Items]
Leverage Ratio	4.50%
Leverage Ratio in Latest Year of Debt Term	3.75%
Notes Payable to Banks [Member] | Minimum [Member] | $200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.25%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.20%
Notes Payable to Banks [Member] | Minimum [Member] | Eurodollar [Member] | $200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.25%
Notes Payable to Banks [Member] | Maximum [Member] | $200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.25%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Notes Payable to Banks [Member] | Maximum [Member] | $825 Million Term B Loan Facility, due June 2022 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate During Period	4.42%
Notes Payable to Banks [Member] | Maximum [Member] | Eurodollar [Member] | $200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.25%
Notes Payable to Banks [Member] | Maximum [Member] | Eurodollar [Member] | $825 Million Term B Loan Facility, due June 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.00%
$200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity | $	$ 186.1