Shareholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions					9 Months Ended
	Oct. 16, 2017	Sep. 15, 2017	Aug. 16, 2017	Aug. 01, 2017	Sep. 30, 2017	Aug. 24, 2017	Dec. 31, 2016	Aug. 01, 2016
Class Of Stock [Line Items]
Common stock, shares issued					91,319,000		85,316,000
Dividends payable, date declared					Oct. 16, 2017
Dividends payable, date to be paid					Nov. 20, 2017
Dividends payable, date of record					Nov. 10, 2017
Subsequent Event [Member]
Class Of Stock [Line Items]
Common stock, dividends declared	$ 0.05
Series C Preferred Stock [Member]
Class Of Stock [Line Items]
Dividend rate percentage					6.50%
Liquidation preference, per share					$ 25.00		$ 25.00
Redemption price per share					$ 25.00
Redemption date					Aug. 24, 2022
Preferred stock, shares issued					3,000,000	3,000,000	0
Preferred stock, issued price per share						$ 25.00
Public offering, realized net proceeds, net offering expenses						$ 72.3
Series C Preferred Stock [Member] | Subsequent Event [Member]
Class Of Stock [Line Items]
Preferred stock dividends declared	0.388194
Series B Preferred Stock [Member]
Class Of Stock [Line Items]
Dividend rate percentage					7.25%
Liquidation preference, per share					$ 25.00		$ 25.00
Redemption price per share		$ 25.00	$ 25.00		$ 25.00
Redemption date					May 22, 2017
Preferred stock, shares issued					3,450,000		7,950,000
Preferred stock redeemed, shares		3,000,000	1,500,000
Redemption price plus accrued and unpaid dividends amount		$ 75.0	$ 37.5
Series B Preferred Stock [Member] | Subsequent Event [Member]
Class Of Stock [Line Items]
Preferred stock dividends declared	$ 0.453125
Forward Sales Agreements [Member]
Class Of Stock [Line Items]
Common stock, shares issued								5,750,000
Public offering, estimated net proceeds				$ 43.2
Settlement date of agreements				Aug. 01, 2017	Aug. 01, 2017