Debt (Narrative) (Details) - USD ($) shares in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt	$ 0	$ 2,300,000	$ 0	$ 1,800,000
Senior revolving credit facility, maximum borrowing capacity	3,500,000,000		3,500,000,000
Equity Unit Purchase Agreements [Member]
Debt Instrument [Line Items]
Equity Units Issued					25.0
Outstanding principal amount	1,250,000,000		1,250,000,000
Equity Units, stated value per unit (whole dollars)	$ 50		$ 50
Equity Units, ownership interest percentage in remarketable subordinated notes	5.00%		5.00%
Par value, remarketable subordinated notes	$ 1,000		$ 1,000
Interest rate, remarketable subordinated notes	1.90%		1.90%
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives			On May 1, 2018, if the applicable market value of our common stock is equal to or greater than $207.6851 per share, the settlement rate will be 0.2406 shares of our common stock. If the applicable market value of our common stock is less than $207.6851 per share but greater than $143.7820 per share, the settlement rate will be a number of shares of our common stock equal to $50 (whole dollars) divided by the applicable market value of our common stock. If the applicable market value of common stock is less than or equal to $143.7820, the settlement rate will be 0.3478 shares of our common stock. Holders of the Equity Units may elect early settlement at a minimum settlement rate of 0.2406 shares of our common stock for each purchase contract being settled.
Stock purchase contract liability, present value	$ 72,100,000		$ 72,100,000
Equity Units, contract adjustment payment rate	3.35%		3.35%
Convertible Debt [Member]
Debt Instrument [Line Items]
Outstanding principal amount	$ 513,400,000		$ 513,400,000
Bridge Loan [Member]
Debt Instrument [Line Items]
Bridge facility fees					$ 106,600,000
Amoritization of bridge facility fees	19,000,000		82,100,000
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank, advances, short-term	$ 440,000,000		$ 440,000,000
Debt instrument interest rate	0.404%		0.404%
Surplus Notes [Member] | Nine Point Zero Zero Zero Percent Due Two Thousand Twenty Seven [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 24,900,000		$ 24,900,000
Remarketable Subordinated Notes [Member] | One Point Nine Zero Zero Percent Due Two Thousand Twenty Eight [Member]
Debt Instrument [Line Items]
Long-term Debt	1,237,300,000		1,237,300,000
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Long-term Debt	13,070,100,000		13,070,100,000
Commercial Paper Program
Debt Instrument [Line Items]
Commercial paper authorized	2,500,000,000		2,500,000,000
Commercial paper	$ 504,700,000		$ 504,700,000