Debt - Credit Facilities (Details) - USD ($) $ in Millions	Dec. 19, 2014	Dec. 31, 2017	Dec. 31, 2016
Line of Credit
Line of Credit Facility [Line Items]
Line of credit outstanding			$ 0.0
Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	$ 400.0
Percentage of loan availability from pledged assets from mortgaged assets, maximum	10.00%
Letters of credit outstanding		$ 64.4
Revolving Credit Facility | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	$ 400.0
Letter of credit sublimit | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	50.0
Letters of credit outstanding		41.8
Swingline Line of Credit | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	50.0
Option to increase maximum borrowing capacity | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	$ 250.0
Letter of Credit | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity		$ 64.5
LIBOR | Minimum | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate basis	2.50%
LIBOR | Maximum | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate basis	3.50%
Applicable Margin, Less Than Or Equal To 35% Utilization | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Facility utilization percentage	35.00%
Applicable Margin, Less Than Or Equal To 35% Utilization | LIBOR | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate basis	2.50%
Applicable Margin, Greater Than 35% But Less Than Or Equal To 50% Utilization | Minimum | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Facility utilization percentage	35.00%
Applicable Margin, Greater Than 35% But Less Than Or Equal To 50% Utilization | Maximum | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Facility utilization percentage	50.00%
Applicable Margin, Greater Than 35% But Less Than Or Equal To 50% Utilization | LIBOR | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate basis	3.25%
Applicable Margin, Greater Than 50% Utilization | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Facility utilization percentage	50.00%
Applicable Margin, Greater Than 50% Utilization | LIBOR | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate basis	3.50%
Unused Commitment Fee, Outstanding Debt Percentage Greater Than Or Equal To 50% | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Quarterly commitment fee	0.25%
Line of credit percentage outstanding	50.00%
Unused Commitment Fee, Outstanding Debt Percentage Less Than 50% | Line of Credit | Fourth Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Quarterly commitment fee	0.35%
Line of credit percentage outstanding	50.00%