Debt Obligations (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Oct. 30, 2014	Apr. 09, 2014
Debt Instrument [Line Items]
Line of Credit Facility, Fair Value of Amount Outstanding	$ 266,738		$ 266,738		$ 54,094
Convertible Debt	377,863		377,863		369,259
Term Loan A	0		0		51,094
Revolving Credit Facility	$ 266,738		$ 266,738		3,000
Revolving Credit Facility, Interest Rate at Period End	3.58%		3.58%
Debt Instrument, Face Amount						$ 402,500
Debt Instrument, Convertible, Conversion Price						$ 72.18
Other Debt	$ 33,696		$ 33,696		27,763
Debt, Long-term and Short-term, Combined Amount	878,297		878,297		451,116
Unamortized Debt Issuance Expense	3,835		3,835		5,816
Debt, net of debt issuance costs	874,462		874,462		445,300
Short-term debt obligations and current maturities of long-term debt obligations	233,074		233,074		41,288
Debt obligations, net of current portion	$ 641,388		641,388		404,012
Line of Credit Facility, Expiration Date	Oct. 17, 2023
Line of Credit Facility, Increase (Decrease), Net	$ 100,000
Minimum [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.375%
Minimum [Member] | Base Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.375%
Maximum [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.375%
Maximum [Member] | Base Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.375%
ATM Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Fair Value of Amount Outstanding	$ 200,000		200,000		$ 0
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases	$ 300,000		$ 300,000
Debt Instrument, Description of Variable Rate Basis			Overnight LIBOR rate plus 1.75%
Line of Credit Facility, Expiration Date			Nov. 30, 2018
Short-term Debt, Weighted Average Interest Rate, over Time	3.70%		3.60%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 600,000		$ 600,000
Term Loan A [Member]
Debt Instrument [Line Items]
Term Loan A							$ 75,000
Senior Secured Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	675,000		$ 675,000
Credit Facility Expiration Date			Apr. 09, 2019
1.5% Issue [Member]
Debt Instrument [Line Items]
Interest Expense, Debt	1,500	$ 1,500	$ 4,500	$ 4,500
Amortization of Debt Discount (Premium)	$ 2,900	$ 2,800	$ 8,600	$ 8,200
Debt Instrument, Interest Rate During Period	4.70%		4.70%
Debt Instrument, Unamortized Discount	$ 24,600		$ 24,600