Shareholders' Equity (Narrative) (Details) $ / shares in Units, $ in Millions	1 Months Ended			3 Months Ended								12 Months Ended
	Jan. 31, 2021 $ / shares	May 31, 2020 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) $ / shares shares	Dec. 31, 2020 $ / shares	Sep. 30, 2020 $ / shares	Jun. 30, 2020 $ / shares	Mar. 31, 2020 $ / shares	Dec. 31, 2019 $ / shares	Sep. 30, 2019 $ / shares	Jun. 30, 2019 $ / shares	Mar. 31, 2019 $ / shares	Dec. 31, 2020 $ / shares	Dec. 31, 2019 $ / shares	Dec. 31, 2018 $ / shares
Shareholders' Equity [Line Items]
Cash dividends declared per common share (usd per share)				$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 1.12	$ 1.00	$ 0.76
Subsequent event
Shareholders' Equity [Line Items]
Cash dividends declared per common share (usd per share)	$ 0.31
Common stock dividends per share declared annualized (in usd per share)	$ 1.24
Series D preferred stock
Shareholders' Equity [Line Items]
Preferred stock, liquidation value per share (usd per share)			$ 25	25				$ 25				25	$ 25
Preferred stock, dividend rate (as a percent)			6.50%
Series D preferred stock | Public Offering
Shareholders' Equity [Line Items]
Stock issued and sold (in shares) | shares			5,000,000
Consideration received on sale | $			$ 125.0
Series E Preferred Stock
Shareholders' Equity [Line Items]
Preferred stock, liquidation value per share (usd per share)		$ 25,000		$ 25,000								$ 25,000
Preferred stock, dividend rate (as a percent)		6.875%
Series E Preferred Stock | Public Offering
Shareholders' Equity [Line Items]
Stock issued and sold (in shares) | shares		11,500
Consideration received on sale | $		$ 287.5
Series E Depositary Shares
Shareholders' Equity [Line Items]
Ratio of preferred stock to depositary shares		0.001
Series E Depositary Shares | Public Offering
Shareholders' Equity [Line Items]
Stock issued and sold (in shares) | shares		11,500,000
London Interbank Offered Rate (LIBOR) | Series D preferred stock
Shareholders' Equity [Line Items]
Variable rate (as a percent)			4.06%