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Long-Term Debt (Details Textuals) - USD ($) $ in Millions		4 Months Ended	12 Months Ended	15 Months Ended	20 Months Ended
Long-Term Debt (Details Textuals) - USD ($) $ in Millions	Jul. 21, 2016	Jul. 21, 2016	Jul. 31, 2019	Mar. 14, 2016	Jul. 21, 2016	Mar. 15, 2016	Dec. 03, 2014
Line of Credit Facility [Line Items]
Total Consolidated Net Leverage Ratio			30.00%
Minimum Liquidity			$ 1,000.0
Applicable interest rate added to reference rate in order to compute variable interest rate			1.00%
Reference rate			2.22%
Federal Funds Rate
Line of Credit Facility [Line Items]
Applicable interest rate added to reference rate in order to compute variable interest rate			0.50%
LIBOR
Line of Credit Facility [Line Items]
Applicable interest rate added to reference rate in order to compute variable interest rate			1.00%
Adjusted LIBOR One \| Minimum
Line of Credit Facility [Line Items]
Applicable interest rate added to reference rate in order to compute variable interest rate		0.125%
Adjusted LIBOR One \| Maximum
Line of Credit Facility [Line Items]
Applicable interest rate added to reference rate in order to compute variable interest rate		1.00%
Adjusted LIBOR Two \| Revolving Credit Facility \| Minimum
Line of Credit Facility [Line Items]
Applicable interest rate added to reference rate in order to compute variable interest rate		1.125%
Line of Credit \| Revolving Credit Facility \| Minimum
Line of Credit Facility [Line Items]
Commitment fee rate range		0.15%
Line of Credit \| Revolving Credit Facility \| Maximum
Line of Credit Facility [Line Items]
Commitment fee rate range		0.30%
Scenario 1 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Covenant Terms 1			325.00%
Note Agreement, Covenant Terms			325.00%
Scenario 2 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Covenant Terms 1			350.00%
Note Agreement, Covenant Terms			350.00%
Wells Fargo and Bank of America
Line of Credit Facility [Line Items]
Maturity date		Mar. 15, 2021
Wells Fargo and Bank of America \| Adjusted LIBOR Two \| Revolving Credit Facility \| Maximum
Line of Credit Facility [Line Items]
Applicable interest rate added to reference rate in order to compute variable interest rate		2.00%
Wells Fargo and Bank of America \| Line of Credit
Line of Credit Facility [Line Items]
Repayments of lines of credit	$ 242.5
Wells Fargo and Bank of America \| Line of Credit \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity							$ 300.0
Wells Fargo and Bank of America \| Line of Credit \| Term Loan Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity							300.0
Amortization of financing costs		$ 7.5		$ 18.8
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. [Member]
Line of Credit Facility [Line Items]
Maturity date			Jul. 21, 2021
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. [Member] \| Revolving Credit Facility \| Minimum
Line of Credit Facility [Line Items]
Commitment fee rate range			0.125%
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. [Member] \| Revolving Credit Facility \| Maximum
Line of Credit Facility [Line Items]
Commitment fee rate range			0.20%
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. [Member] \| Scenario 3 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Covenant Terms			not to exceed $50.0 million
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. [Member] \| Scenario 4 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Covenant Terms			not less than $75.0 million
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. [Member] \| Adjusted LIBOR One \| Revolving Credit Facility \| Minimum
Line of Credit Facility [Line Items]
Applicable interest rate added to reference rate in order to compute variable interest rate			0.00%
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. [Member] \| Adjusted LIBOR One \| Revolving Credit Facility \| Maximum
Line of Credit Facility [Line Items]
Applicable interest rate added to reference rate in order to compute variable interest rate			0.75%
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. [Member] \| Adjusted LIBOR Two \| Revolving Credit Facility \| Minimum
Line of Credit Facility [Line Items]
Applicable interest rate added to reference rate in order to compute variable interest rate			1.00%
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. [Member] \| Adjusted LIBOR Two \| Revolving Credit Facility \| Maximum
Line of Credit Facility [Line Items]
Applicable interest rate added to reference rate in order to compute variable interest rate			1.75%
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. [Member] \| Line of Credit \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Outstanding borrowings			$ 0.0
First Amendment To Credit Agreement \| Wells Fargo and Bank of America \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity						$ 350.0
Line of Credit, Maximum Borrowing Capacity, Increase						50.0
First Amendment To Credit Agreement \| Wells Fargo and Bank of America \| Term Loan Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity						93.8
Outstanding borrowings						$ 93.8
Credit Agreement \| Term Loan Facility
Line of Credit Facility [Line Items]
Maturity date		Dec. 03, 2019
Second Amendment To Credit Agreement \| WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. [Member] \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	500.0	$ 500.0	$ 850.0		$ 500.0
Note Purchase Agreement
Line of Credit Facility [Line Items]
Debt instrument costs					3.4
Capitalized debt issuance fees	2.0	2.0			2.0
Debt discount	$ 1.4	$ 1.4			$ 1.4
Note Purchase Agreement \| Senior Notes
Line of Credit Facility [Line Items]
Senior notes							400.0
4.07% Senior Notes, Series A \| Senior Notes
Line of Credit Facility [Line Items]
Senior notes							$ 100.0
Stated interest rate							4.07%
4.19% Senior Notes, Series B \| Senior Notes
Line of Credit Facility [Line Items]
Senior notes							$ 100.0
Stated interest rate							4.19%
4.25% Senior Notes, Series C \| Senior Notes
Line of Credit Facility [Line Items]
Senior notes							$ 100.0
Stated interest rate							4.25%
4.35% Senior Notes, Series D \| Senior Notes
Line of Credit Facility [Line Items]
Senior notes							$ 100.0
Stated interest rate							4.35%