Stock-based Compensation Expense Activity in Performance Unit Awards (Details) - Performance Unit Awards	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Shares
Nonvested, beginning of period (in shares) | shares	494,691
Granted (in shares) | shares	82,396
Vested (in shares) | shares	(131,374)
Forfeited (in shares) | shares	(12,710)
Nonvested, end of period (in shares) | shares	433,003
Weighted Average Grant Date Fair Value
Nonvested, beginning of period (in dollars per share) | $ / shares	$ 66.42
Granted (in dollars per share) | $ / shares	116.43
Vested (in dollars per share) | $ / shares	67.59
Forfeited (in dollars per share) | $ / shares	67.36
Nonvested, end of period (in dollars per share) | $ / shares	$ 75.55