Summary of Significant Accounting Policies - Stock Compensation Expense (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Stock compensation expense	$ 4,184,000	$ 3,506,000	$ 2,719,000
Capitalized stock compensation	604,000	485,000	386,000
Stock compensation expense, net	3,580,000	3,021,000	2,333,000
Excess Tax Benefits, net of deficiencies	$ 3,314	$ 1,709,000	$ 679,000