Debt, Line of Credit Facility (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Line of Credit [Member]
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	$ 400.0
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 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	0.25%
Line of credit outstanding	$ 0.0
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	32.4
Letters of Credit Outstanding, Amount	64.5	$ 64.4
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	0.0
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	$ 400.0