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Other Benefit Plans (Schedule of Employees Eligible to Participate in Employee Stock Purchase Plan) (Detail) - $ / shares
6 Months Ended 12 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Pension Plans And Defined Benefit Postretirement Plans Disclosure [Abstract]          
ESPP shares purchased 1,940 2,289 4,465 4,162 3,432
Average purchase price per share     $ 18.520 $ 15.865 $ 15.318