Debt (Narrative) (Details) - USD ($) shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 10, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt	$ 2,300,000	$ (74,800,000)	$ 1,800,000	$ (80,800,000)
Senior revolving credit facility, maximum borrowing capacity	3,500,000,000		3,500,000,000
Repayments of Long-term Debt			(2,386,500,000)	(1,730,800,000)
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)	(110,600,000)	$ (36,700,000)	(110,600,000)	$ (36,700,000)
Terminated Credit Facility [Domain]
Debt Instrument [Line Items]
Senior revolving credit facility, maximum borrowing capacity	2,000,000,000		2,000,000,000
Forward Starting Pay Fixed Swaps [Member]
Debt Instrument [Line Items]
Derivative, Notional Amount	2,750,000,000		2,750,000,000
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)	110,600,000		$ 110,600,000			$ 35,900,000
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
Net proceeds from issuance of Equity Units	$ 1,228,800,000
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 $207.805 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 $207.805 per share but greater than $143.865 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 $143.865, the settlement rate will be 0.3475 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	$ 112,300,000		$ 112,300,000
Equity Units, contract adjustment payment rate	3.35%		3.35%
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Senior unsecured notes, carrying value	$ 13,054,400,000		$ 13,054,400,000
Other senior unsecured notes [Member]
Debt Instrument [Line Items]
Repurchased Face Amount			13,000,000
Senior unsecured notes, repurchase price			16,200,000
Gain (loss) on extinguishment of debt			$ (3,400,000)
Remarketable Subordinated Notes [Member]
Debt Instrument [Line Items]
Equity Units, ownership interest percentage in remarketable subordinated notes	5.00%		5.00%
Interest rate, remarketable subordinated notes	1.90%		1.90%
Surplus Notes [Member]
Debt Instrument [Line Items]
Unsecured surplus note, carrying amount	$ 24,900,000		$ 24,900,000
Convertible Debt [Member]
Debt Instrument [Line Items]
Repurchased Face Amount	50,000,000		750,500,000
Gain (loss) on extinguishment of debt	2,300,000		5,200,000
Outstanding principal amount	683,000,000		683,000,000
Aggregate principal amount of convertible debentures surrendered for conversion	100,000		66,600,000
Repayments of Long-term Debt	98,700,000		1,745,000,000
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank, advances, short-term	$ 540,000,000		$ 540,000,000
Debt instrument interest rate	0.374%		0.374%
Senior Unsecured Notes [Member] | One Point Two Five Zero Percent Due Two Thousand Fifteen [Member]
Debt Instrument [Line Items]
Debt instruments, repurchased face amount					$ 625,000,000
Interest rate, remarketable subordinated notes	1.25%		1.25%			1.25%
Senior Unsecured Notes [Member] | Two Point Two Five Zero Percent Due Two Thousand Nineteen [Member]
Debt Instrument [Line Items]
Interest rate, remarketable subordinated notes	2.25%		2.25%			2.25%
Senior Unsecured Notes [Member] | Three Point Five Zero Zero Percent Due Two Thousand Twenty Four [Member]
Debt Instrument [Line Items]
Interest rate, remarketable subordinated notes	3.50%		3.50%			3.50%
Senior Unsecured Notes [Member] | Four Point Six Five Zero Percent Due Two Thousand Forty Four [Member]
Debt Instrument [Line Items]
Interest rate, remarketable subordinated notes	4.65%		4.65%			4.65%
Senior Unsecured Notes [Member] | Four Point Eight Five Zero Percent Due Two Thousand Fifty Four [Member]
Debt Instrument [Line Items]
Interest rate, remarketable subordinated notes	4.85%		4.85%			4.85%
Commercial Paper Program
Debt Instrument [Line Items]
Commercial paper authorized	$ 2,500,000,000		$ 2,500,000,000
Commercial paper	$ 965,800,000		$ 965,800,000