Debt, Line of Credit (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2013	Dec. 31, 2014
Credit Facilities [Line Items]
Line of credit	$ 270,000		$ 100,000
Line of Credit [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	$ 500,000	$ 250,000
Maturity date	Jan. 03, 2020	Mar. 31, 2018
Description of applicable margin calculation based on utilization percentage	Amounts drawn under the facility bear interest at 90-day LIBOR plus an applicable margin; however, the amended agreement reduces the applicable margin from a range of 3.25% to 4.25% to a range of 2.50% to 3.50%. The applicable margin varies based on the percentage of the total commitment draw, 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%.
Quarterly commitment fee	0.50%
Line of credit	$ 270,000
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	111,800
Letters of Credit Outstanding, Amount	83,700
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