Earnings per Share (Details) - $ / shares	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)	$ 0.84	$ 0.77	$ 0.74	$ 0.83	$ 0.76	$ 0.74	$ 0.70	$ 0.67	$ 0.65	$ 0.59	$ 0.54	$ 0.51	$ 3.19	$ 2.88	$ 2.29
Diluted earnings per common share (in dollars per share)	$ 0.84	$ 0.76	$ 0.74	$ 0.83	$ 0.76	$ 0.74	$ 0.70	$ 0.67	$ 0.65	$ 0.59	$ 0.54	$ 0.51	$ 3.17	$ 2.86	$ 2.28
Basic weighted average shares outstanding (in shares)													31,552,000	32,603,000	34,668,000
Diluted weighted average shares outstanding (in shares)													31,719,000	32,863,000	34,901,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount													0	0	0
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Basic weighted average shares outstanding (in shares)													31,552,000	32,603,000	34,668,000
Incremental shares for common stock equivalents (in shares)													167,000	260,000	233,000
Diluted shares outstanding (in shares)													31,719,000	32,863,000	34,901,000