Stockholders' Equity (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shares repurchased	$ 3,431,900,000
Dividends	113,700,000	$ 123,000,000	$ 114,900,000
Total comprehensive income (loss)	521,800,000	(846,000,000)	1,025,900,000
Accumulated other comprehensive loss	86,500,000	140,100,000
Reclassification adjustments impacting net income	23,200,000	6,300,000
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	8,000,000	61,400,000
Reclassification adjustments impacting net income	26,200,000	4,700,000	80,500,000
Change in income taxes related to other comprehensive income (loss)	24,000,000	3,100,000	(35,900,000)
Changes in benefit plan net gain (loss) and prior service (cost) credit
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	54,500,000	65,300,000
Reclassification adjustments impacting net income	700,000	1,600,000	2,000,000
Change in income taxes related to other comprehensive income (loss)	(6,200,000)	(1,700,000)	6,800,000
Unrealized net gains (losses) on available for sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	24,000,000	13,400,000
Reclassification adjustments impacting net income	(3,700,000)	0
Change in income taxes related to other comprehensive income (loss)	6,900,000	$ 4,300,000	$ 4,300,000
Common Share Repurchases
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shares repurchased	$ 3,400,000,000
Equity Tender Offer
Accumulated Other Comprehensive Income (Loss) [Line Items]
Share repurchased (in shares)	57,300,000
Average share price of shares repurchased (in shares)	$ 48.00
Open Market Repurchases
Accumulated Other Comprehensive Income (Loss) [Line Items]
Share repurchased (in shares)	3,600,560
Average share price of shares repurchased (in shares)	$ 45.27
Accelerated Share Repurchase Program, Initial Settlement
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shares repurchased	$ 512,000,000
Share repurchased (in shares)	9,253,668
Accelerated Share Repurchase Program, Final Settlement
Accumulated Other Comprehensive Income (Loss) [Line Items]
Share repurchased (in shares)	1,452,119
Average share price of shares repurchased (in shares)	$ 47.82