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Employee Benefit Plans Summary of ARX employee stock option plans (Details) - Employee Stock Option - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Apr. 01, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number 22,550 24,995 24,995 26,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price $ 564.60 $ 526.46 $ 526.46 $ 513.72
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (2,445) 0 (1,005)  
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price $ 174.65 $ 0 $ 197.01  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Number 17,950 16,995 12,995  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price $ 517.75 $ 438.77 $ 386.69  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value $ 2.9   $ 1.1  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares 4,600 8,000 12,000 14,800
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value $ 747.45 $ 712.74 $ 677.81 $ 675.55
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares (3,400) (4,000) (2,800)  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Weighted Average Grant Date Fair Value $ 665.79 $ 607.95 $ 665.85  
Unrecognized compensation cost related to unvested equity awards $ 0.7 $ 1.6 $ 2.9  
Period of recognition of compensation expense related to unvested equity awards 1 year 5 days 1 year 4 months 10 days 1 year 8 months 20 days