Long-Term Debt (Details) - USD ($) $ in Millions | Jul. 21, 2016 | Jul. 21, 2016 | Apr. 30, 2020 | Mar. 14, 2016 | Apr. 30, 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. 3, 2019 | | | | | |
Note Purchase Agreement | Senior Notes | | | | | | | |
Line of Credit Facility [Line Items] | | | | | | | |
Debt Instrument, Face Amount | | | | | | | $ 400 |
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 |
Senior Notes, Series B [Member] | Senior Notes | | | | | | | |
Line of Credit Facility [Line Items] | | | | | | | |
Debt Instrument, Face Amount | | | | | | | 100 |
Senior Notes, Series C [Member] | Senior Notes | | | | | | | |
Line of Credit Facility [Line Items] | | | | | | | |
Debt Instrument, Face Amount | | | | | | | 100 |
Senior Notes, Series D [Member] | Senior Notes | | | | | | | |
Line of Credit Facility [Line Items] | | | | | | | |
Debt Instrument, Face Amount | | | | | | | 100 |
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 |
Wells Fargo and Bank of America, N.A. | Credit Agreement | Term Loan Facility | | | | | | | |
Line of Credit Facility [Line Items] | | | | | | | |
Maximum borrowing capacity | | | | | | | $ 300 |
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 | |
Line of Credit, Maximum Borrowing Capacity, Increase | | | | | | 50 | |
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 | | | $ 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 | | | LIBOR Rate of 0.33% plus an applicable margin of 1.00%. | | | | |
WellsFargo,NationalAssociation,SunTrustBankandBankofAmerica,N.A. | Second Amendment To Credit Agreement | Revolving Credit Facility | | | | | | | |
Line of Credit Facility [Line Items] | | | | | | | |
Maximum borrowing capacity | $ 500 | $ 500 | | | $ 850 | | |
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% | | | | |