Share-based Compensation (Nonvested Restricted Stock) (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Installment $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Nonvested Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Common shares reserved for stock option and restricted stock grants	5,363,000.000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rate, initial year (percent)	33.33%
Quarterly award vesting installments after initial year | Installment	8
Nonvested Awards, Number of Shares [Roll Forward]
Nonvested awards, Shares, beginning balance	123,654	203,314	188,522
Nonvested awards, Shares Granted	56,251	26,119	132,069
Nonvested awards, Shares Vested	(72,810)	(101,441)	(108,798)
Nonvested awards, Shares Forfeited	0	(4,338)	(8,479)
Nonvested awards, Shares, ending balance	107,095	123,654	203,314
Nonvested Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Nonvested awards outstanding, Weighted Average Grant Date Fair Value, beginning balance (in dollars per share) | $ / shares	$ 31.17	$ 25.56	$ 17.13
Nonvested awards, Granted, Weighted Average Grant Date Fair Value (in dollars per share) | $ / shares	56.75	44.79	30.29
Nonvested awards, Vested, Weighted Average Grant Date Fair Value (in dollars per share) | $ / shares	29.50	23.25	17.35
Nonvested awards, Forfeited, Weighted Average Grant Date Fair Value (in dollars per share) | $ / shares	0.00	35.54	17.20
Nonvested awards outstanding, Weighted Average Grant Date Fair Value, ending balance (in dollars per share) | $ / shares	$ 45.74	$ 31.17	$ 25.56
Unrecognized compensation cost, share-based compensation arrangements | $	$ 4.2
Weighted average period to recognize nonvested awards (in years)	2 years
Fair value of shares vested during period | $	$ 2.1	$ 2.4	$ 1.9
Stock compensation expense recognized on restricted grants | $	$ 2.4	$ 2.5	$ 1.5