Share-based Compensation - Nonvested Restricted Stock (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) Installment $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Nonvested Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Common shares reserved for stock option and restricted stock grants	5,200,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rate, initial year	33.33%
Quarterly award vesting installments after initial year | Installment	8
Nonvested Awards, Number of Shares [Roll Forward]
Nonvested awards, beginning balance (in shares)	40,416	107,095	123,654
Granted (in shares)	91,658	5,623	56,251
Vested (in shares)	(24,120)	(68,788)	(72,810)
Forfeited (in shares)	0	(3,514)	0
Nonvested awards, ending balance (in shares)	107,954	40,416	107,095
Nonvested Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Nonvested awards outstanding, beginning balance (in dollars per share) | $ / shares	$ 58.60	$ 45.74	$ 31.17
Granted (in dollars per share) | $ / shares	50.54	76.47	56.75
Vested (in dollars per share) | $ / shares	57.14	40.67	29.50
Forfeited (in dollars per share) | $ / shares	0	46.24	0
Nonvested awards outstanding, ending balance (in dollars per share) | $ / shares	$ 52.08	$ 58.60	$ 45.74
Unrecognized compensation cost, share-based compensation arrangements | $	$ 3.9
Weighted average period to recognize nonvested awards (in years)	1 year 9 months 7 days
Fair value of shares vested during period | $	$ 1.4	$ 2.8	$ 2.1
Stock compensation expense recognized on restricted grants | $	$ 2.9	$ 2.4	$ 2.4