Employee Benefits (Details 3) - Restricted Shares Subject to Service and Performance Conditions - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted shares subject only to a service condition:
Non-vested, January 1, 2015 (in shares)	153,620
Granted (in shares)	160,340
Vested (in shares)	(102,367)
Forfeited (in shares)	(45,675)
Non-vested, December 31, 2015 (in shares)	165,918	153,620
Non-vested, January 1, 2015 (in dollars per share)	$ 55.90
Granted (in dollars per share)	54.86	$ 55.90	$ 61.29
Vested (in dollars per share)	56.58
Forfeited (in dollars per share)	55.86
Non-vested, December 31, 2015 (in dollars per share)	$ 55.85	$ 55.90