Debt, Line of Credit Facility (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2013
Line of Credit [Member]
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	$ 500.0	$ 250.0
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%
Credit facilities borrowings outstanding	$ 310.0
Letters of Credit Outstanding, Amount	19.4
Swingline Line of Credit [Member]
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	50.0
Secured and Unsecured Letter of Credit Facilities
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	80.2
Letters of Credit Outstanding, Amount	63.0
Letter of credit sublimit [Member]
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	50.0
Option to increase maximum borrowing capacity [Member]
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	250.0
Term Loan [Member]
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	100.0
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	$ 400.0