Employee Benefits (Details 1) - Restricted Shares Subject Only to a Service Condition	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Restricted shares subject only to a service condition:
Non-vested, January 1, 2015 (in shares) | shares	607,501
Granted (in shares) | shares	143,460
Vested (in shares) | shares	(187,295)
Forfeited (in shares) | shares	(44,291)
Non-vested, December 31, 2015 (in shares) | shares	519,375
Non-vested, January 1, 2015 (in dollars per share) | $ / shares	$ 54.67
Granted (in dollars per share) | $ / shares	61.20
Vested (in dollars per share) | $ / shares	57.47
Forfeited (in dollars per share) | $ / shares	54.99
Non-vested, December 31, 2015 (in dollars per share) | $ / shares	$ 55.44