Stock-Based Compensation (Performance Share Awards Activity) (Details) - Performance shares	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Performance Shares
Balance at beginning of period (in shares) | shares	1,392,512
Grants (in shares) | shares	494,688
Forfeitures (in shares) | shares	(49,547)
Payouts (in shares) | shares	(513,252)
Balance at end of period (in shares) | shares	1,324,401
Weighted Average Grant Date Fair Value
Balance at beginning of period (in dollars per share) | $ / shares	$ 79.97
Grants (in dollars per share) | $ / shares	98.47
Forfeitures (in dollars per share) | $ / shares	87.12
Payouts (in dollars per share) | $ / shares	70.60
Balance at end of period (in dollars per share) | $ / shares	$ 90.31