Common Stock - Stock Options Status and Activity (Details) - USD ($)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Total fair value of shares vested				$ 0	$ 2,000,000
Shares Under Option
Outstanding, Beginning Balance (in shares)	311,041	313,409	318,016	318,016
Exercised (in shares)	(11,420)		(4,607)
Forfeited (in shares)		(2,368)
Outstanding, Ending Balance (in shares)	299,621	311,041	313,409	299,621		318,016
Weighted Avg. Exercise Prices
Outstanding, Beginning Balance (in dollars per share)	$ 43.76	$ 43.84	$ 43.60	$ 43.60
Exercised (in dollars per share)	29.93		26.78
Forfeited (in dollars per share)		54.34
Outstanding, Ending Balance (in dollars per share)	$ 44.29	$ 43.76	$ 43.84	$ 44.29		$ 43.60
Weighted Avg. Remaining Life in Years		1 year 9 months 18 days	2 years 1 month 6 days	2 years 5 months 6 days		2 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding, Beginning Balance	$ 2,000,000	$ 4,000,000	$ 5,000,000	$ 5,000,000
Aggregate Intrinsic Value, Exercised	0	0	0
Aggregate Intrinsic Value, Outstanding, Ending Balance	$ 1,000,000	$ 2,000,000	$ 4,000,000	$ 1,000,000		$ 5,000,000