Stockholders' Equity (Narrative) (Details) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) shares
Class of Stock [Line Items]
Distributable reserves	$ 10,100.0
Stock repurchase program, authorized amount	$ 500.0
Shares repurchased (in shares) | shares	3
Shares repurchased, value	$ 92.6