Employee Benefit Plans (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) installment $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares
Defined Benefit Plan Disclosure
Employer contribution for defined contribution plan	$ 2,000,000	$ 1,000,000	$ 900,000
Employee Stock Ownership Plan, common stock, shares outstanding (shares) | shares	149,050.91
Employee Stock Ownership Plan, common stock, fair value	$ 14,600,000
Outside Directors
Defined Benefit Plan Disclosure
Number of deferred common stock (shares) | shares	10,166	8,466	9,415
Weighted-average grant date fair values of deferred common stock (dollars per share) | $ / shares	$ 97.99	$ 78.62	$ 70.63
Nonqualified Deferred Compensation Plan
Defined Benefit Plan Disclosure
Deferred compensation expense	$ 96,000	$ 106,000	$ 82,000
Number of annual installments of deferred compensation (installment) | installment	2
Number of days after departure from board of directors for payment of deferred compensation (days)	90 days