Long-Term Debt (Details) - USD ($) $ in Millions		4 Months Ended	6 Months Ended	15 Months Ended
	Jul. 21, 2016	Jul. 21, 2016	Jan. 31, 2020	Mar. 14, 2016	Jul. 31, 2019	Jan. 31, 2017	Mar. 15, 2016	Dec. 03, 2014
Line of Credit Facility [Line Items]
Maturity date		Mar. 15, 2021
Scenario 3
Line of Credit Facility [Line Items]
Long-term Debt			not to exceed $50.0 million
Scenario 4
Line of Credit Facility [Line Items]
Long-term Debt			not less than $75.0 million
Credit Agreement | Term Loan Facility
Line of Credit Facility [Line Items]
Maturity date		Dec. 03, 2019
Note Purchase Agreement | Senior Notes
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount								$ 400.0
Note Purchase Agreement | Senior Notes, Series A
Line of Credit Facility [Line Items]
Debt instrument, interest rate								4.07%
Note Purchase Agreement | Senior Notes, Series B
Line of Credit Facility [Line Items]
Debt instrument, interest rate								4.19%
Note Purchase Agreement | Senior Notes, Series C
Line of Credit Facility [Line Items]
Debt instrument, interest rate								4.25%
Note Purchase Agreement | Senior Notes, Series D
Line of Credit Facility [Line Items]
Debt instrument, interest rate								4.35%
Senior Notes, Series A [Member] | Senior Notes
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount								$ 100.0
Senior Notes, Series B [Member] | Senior Notes
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount								100.0
Senior Notes, Series C [Member] | Senior Notes
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount								100.0
Senior Notes, Series D [Member] | Senior Notes
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount								100.0
Wells Fargo and Bank of America, N.A. | Line of Credit
Line of Credit Facility [Line Items]
Repayments of Lines of Credit	$ 242.5
Wells Fargo and Bank of America, N.A. | Revolving Credit Facility | Line of Credit | Minimum
Line of Credit Facility [Line Items]
Line of Credit Facility, Commitment Fee Percentage		0.15%
Wells Fargo and Bank of America, N.A. | Revolving Credit Facility | Line of Credit | Maximum
Line of Credit Facility [Line Items]
Line of Credit Facility, Commitment Fee Percentage		0.30%
Wells Fargo and Bank of America, N.A. | Term Loan Facility | Line of Credit
Line of Credit Facility [Line Items]
Quarterly amortization for term loan		$ 7.5
Wells Fargo and Bank of America, N.A. | Credit Agreement | Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity								300.0
Wells Fargo and Bank of America, N.A. | Credit Agreement | Term Loan Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity								$ 300.0
Wells Fargo and Bank of America, N.A. | Credit Agreement | Term Loan Facility | Line of Credit
Line of Credit Facility [Line Items]
Quarterly amortization for term loan				$ 18.8
Wells Fargo and Bank of America, N.A. | First Amendment To Credit Agreement | Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity							$ 350.0
Line of Credit, Maximum Borrowing Capacity, Increase							50.0
Wells Fargo and Bank of America, N.A. | First Amendment To Credit Agreement | Term Loan Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity							93.8
Outstanding borrowings							$ 93.8
Wells Fargo and Bank of America, N.A. | London Interbank Offered Rate (LIBOR), 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%
Wells Fargo and Bank of America, N.A. | London Interbank Offered Rate (LIBOR), 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%
Wells Fargo and Bank of America, N.A. | London Interbank Offered Rate (LIBOR), 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%
Wells Fargo and Bank of America, N.A. | London Interbank Offered Rate (LIBOR), 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%
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A.
Line of Credit Facility [Line Items]
Maturity date			Jul. 21, 2021
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. | Revolving Credit Facility | Minimum
Line of Credit Facility [Line Items]
Line of Credit Facility, Commitment Fee Percentage			0.125%
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. | Revolving Credit Facility | Maximum
Line of Credit Facility [Line Items]
Line of Credit Facility, Commitment Fee Percentage			0.20%
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. | Revolving Credit Facility | Line of Credit
Line of Credit Facility [Line Items]
Outstanding borrowings			$ 13.6		$ 0.0
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. | Scenario 3
Line of Credit Facility [Line Items]
Line of Credit Facility, Covenant Terms			not to exceed $50.0 million
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. | Scenario 4
Line of Credit Facility [Line Items]
Line of Credit Facility, Covenant Terms			not less than $75.0 million
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. | Credit Agreement | Term Loan Facility
Line of Credit Facility [Line Items]
LIBOR variable interest rate			Prime Rate of 4.75% plus an applicable margin of 0.00%.
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. | Second Amendment To Credit Agreement | Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 500.0	$ 500.0				$ 850.0
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. | Federal Funds Effective Swap Rate | Credit Agreement | Revolving Credit Facility
Line of Credit Facility [Line Items]
Applicable interest rate added to reference rate in order to compute variable interest rate			0.50%
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. | London Interbank Offered Rate (LIBOR) | Credit Agreement | Revolving Credit Facility
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. | London Interbank Offered Rate (LIBOR), 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. | London Interbank Offered Rate (LIBOR), 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. | London Interbank Offered Rate (LIBOR), 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. | London Interbank Offered Rate (LIBOR), 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%