Shareholders' Equity (Narrative) (Details) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)	Dec. 31, 2010 shares	Dec. 05, 2012 $ / shares
Schedule of Equity Method Investments [Line Items]
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax		$ (44,898)	$ 48,422
Liquidation preference of preferred stock (per share) | $ / shares		$ 1,000
Preferred stock, authorized shares | shares		4,400,000	4,400,000
Carrying value, Preferred stock		$ 709,601	$ 828,490
Price per share | $ / shares						$ 36.27
Common Stock		$ 4,724,715	$ 4,766,731
Common Stock, Shares, Issued | shares		203,085,100	204,417,093
Warrants sold | shares					29,300,000
Common stock issued for preferred stock redemption		$ 126,144	$ (176,049)
Stock Repurchased During Period, Shares | shares		2,890,000
Stock Repurchased and Retired During Period, Value	$ (45,000)	$ 90,057
Treasury Stock Acquired, Average Cost Per Share | $ / shares	$ 42.43	$ 31.15
Treasury Stock Acquired, Repurchase Authorization	45
Preferred Stock, Tender Offer		$ 120,000
Preferred Stock, Liquidation Preference, Value		119,000
Payments for Repurchase of Redeemable Preferred Stock		126,144	175,669	$ 0
Preferred Stock Redemption		9,800
Total invested assets of trusts		91,000	81,800
Total obligations of trusts		105,200	96,900
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax		(76)	(331)
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax		$ 5,356	374
Preferred stock, liquidation preference per depositary share | $ / shares		$ 25
Common Stock Held in Trust		$ 14,100	15,200
Warrant Exercise Price | $ / shares		$ 35.82
Depositary Share, Preferred Stock Ownership Interest		1/40
Series I Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred stock, authorized shares | shares		300,893,000
Carrying value, Preferred stock		$ 98,555	125,224
Preferred Stock, Dividend Rate, Percentage		5.80%
Common stock issued for preferred stock redemption			176,000
Preferred Stock, Tender Offer			180,000
Series A Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred stock, authorized shares | shares		140,000,000
Variable dividend payment rate		> of 4.0% or 3mL+0.52%
Carrying value, Preferred stock		$ 66,316	66,316
Series F Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred stock, authorized shares | shares		250,000,000
Carrying value, Preferred stock		$ 143,750	143,750
Preferred Stock, Dividend Rate, Percentage		7.90%
Series G Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred stock, authorized shares | shares		200,000,000
Carrying value, Preferred stock		$ 138,391	171,827
Preferred Stock, Dividend Rate, Percentage		6.30%
Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Common stock issued for preferred stock redemption		$ 118,889	(175,669)
Retained Earnings [Member]
Schedule of Equity Method Investments [Line Items]
Common stock issued for preferred stock redemption		9,759	(3,449)
Warrant [Member]
Schedule of Equity Method Investments [Line Items]
Common stock warrants issued to the U.S. Treasury						5,800,000
Common Stock [Member]
Schedule of Equity Method Investments [Line Items]
Common stock issued for preferred stock redemption		$ (2,504)	$ 3,069
Stock Repurchased During Period, Shares | shares		(2,888,946)
Stock Repurchased and Retired During Period, Value		$ 90,057
Series G Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred Stock, Liquidation Preference, Value		33,000
Series I Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred Stock, Liquidation Preference, Value		27,000
SERIES J STOCK [Member]
Schedule of Equity Method Investments [Line Items]
Preferred Stock, Liquidation Preference, Value		$ 59,000