Financing Agreement - Debt Instruments (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Long-term debt	$ 1,112,142,000	$ 425,707,000	$ 200,000,000
Unamortized Debt Issuance Expense	(3,156,000)	(1,803,000)
Total debt, capital lease obligations, and notes payable	1,141,910,000	461,476,000
Less: current maturities	3,847,000	5,059,000
Total long-term debt, capital lease obligations, and notes payable	$ 1,138,063,000	456,417,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Average interest rate	1.66%
Long-term debt	$ 172,142,000	225,707,000
Senior Notes | Senior Unsecured Notes, 3.81 Percent, Due May 2018
Debt Instrument [Line Items]
Long-term debt	25,000,000	25,000,000
Senior Notes | Senior Unsecured Notes, 4.60 Percent, Due May 2021
Debt Instrument [Line Items]
Long-term debt	75,000,000	75,000,000
Senior Notes | Senior Unsecured Notes 3.13 Percent Due December 2020
Debt Instrument [Line Items]
Long-term debt	100,000,000	100,000,000
Capital lease obligations
Debt Instrument [Line Items]
Capital lease obligations	19,306,000	21,874,000
Notes payable | Notes Payable 3.50 Percent Due June 2027
Debt Instrument [Line Items]
Long-term debt	$ 13,618,000	15,698,000
Long-term Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End	1.98%
Long-term Line of Credit	$ 740,000,000	$ 0