Debt, Line of Credit (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Credit Facilities [Line Items]
Line of credit	$ 186,500	$ 310,000
Line of Credit [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	$ 500,000
Maturity date	Jan. 03, 2020
Description of applicable margin calculation based on utilization percentage	Amounts drawn under the facility bear interest at 90-day LIBOR plus an applicable margin from a range of 2.50% to 3.50%. The applicable margin varies based on the percentage of the total commitment drawn, with a 2.50% margin at utilization equal to or lower than 35%, a 3.25% margin at utilization greater than 35% but less than or equal to 50%, and a 3.50% margin at utilization greater than 50%.
Letters of Credit Outstanding, Amount	$ 22,000
Line of credit	186,500
Swingline Line of Credit [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	50,000
Secured and Unsecured Letter of Credit Facilities
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	64,500
Letters of Credit Outstanding, Amount	64,400
Option to increase maximum borrowing capacity [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	250,000
Term Loan [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	100,000
Revolving Credit Facility [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	400,000
Letter of credit sublimit [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	$ 50,000