Forward Equity and Equity Distribution Program (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class of Stock
Proceeds from issuance of common stock		$ 465.8	$ 57.8	$ 41.4
Common stock, shares outstanding (in shares)		58,500,000	55,800,000	55,800,000	53,900,000
Equity Distribution Program
Class of Stock
Maximum number of shares available for sale under agreement (in shares)	2,900,000
Common stock price (in dollars per share)		$ 167.25
Number of common shares issued (in shares)		2,700,000
Proceeds from issuance of common stock		$ 440.3
Number of shares net settled for cash (in shares)		200,000
Average share price of shares net settled for cash (in dollars per share)		$ 144.59
Common stock, shares outstanding (in shares)		0
Stock repurchase program, maximum amount authorized		$ 500.0