Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Statement Of Financial Position [Abstract]
Allowance for doubtful accounts	$ 3,627	$ 3,057
Accumulated depreciation on property, plant and equipment	1,589,422	1,493,705
Accumulated amortization on other assets	$ 25,019	$ 20,737