Debt, Line of Credit (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2013	Dec. 31, 2014
Credit Facilities [Line Items]
Maturity date	Apr. 01, 2020
Line of credit	$ 300,000,000		$ 100,000,000
Line of Credit [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	500,000,000	250,000,000
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	300,000,000
Swingline Line of Credit [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	50,000,000
Secured and Unsecured Letter of Credit Facilities
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	109,300,000
Letters of Credit Outstanding, Amount	83,900,000
Option to increase maximum borrowing capacity [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	250,000,000
Term Loan [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	100,000,000
Revolving Credit Facility [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	400,000,000
Letter of credit sublimit [Member]
Credit Facilities [Line Items]
Credit facility, maximum borrowing capacity	$ 50,000,000