Stock Repurchase Program (Details) - $ / shares	3 Months Ended	6 Months Ended		155 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jul. 26, 2005
Equity [Abstract]
Maximum number of share authorized to repurchase under the program (in shares)					1,500,000
Common stock purchased during the period (in shares)	8,831	56,263	25,727	1,066,181
Average price per share (in dollars per share)	$ 57.36