Stock Options and Restricted Shares - Restricted Share Unit Awards Outstanding (Details)	9 Months Ended
	Sep. 30, 2018 shares
2018 Restricted granted shares | Tranche 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period per tranche (in years)	2 years
Subsequent holding period per tranche (in years)	3 years
2018 Restricted granted shares | Tranche 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period per tranche (in years)	2 years
Subsequent holding period per tranche (in years)	2 years
2018 Restricted granted shares | Tranche 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period per tranche (in years)	2 years
Subsequent holding period per tranche (in years)	1 year
2018 Restricted granted shares | Named Executive Officers (NEOs) | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded (in shares)	96,878	[1]
2018 Restricted granted shares | Named Executive Officers (NEOs) | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded (in shares)	145,339	[1]
2018 Restricted granted shares | Leadership Team other than NEOs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded (in shares)	25,990
2017 Restricted granted shares | Tranche 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period per tranche (in years)	2 years
Subsequent holding period per tranche (in years)	3 years
2017 Restricted granted shares | Tranche 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period per tranche (in years)	2 years
Subsequent holding period per tranche (in years)	2 years
2017 Restricted granted shares | Tranche 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period per tranche (in years)	2 years
Subsequent holding period per tranche (in years)	1 year
2017 Restricted granted shares | Named Executive Officers (NEOs) | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded (in shares)	72,537	[1]
2017 Restricted granted shares | Named Executive Officers (NEOs) | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded (in shares)	109,339	[1]
2017 Restricted granted shares | Leadership Team other than NEOs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded (in shares)	24,916
2016 Restricted granted shares | Tranche 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period per tranche (in years)	2 years
Subsequent holding period per tranche (in years)	3 years
2016 Restricted granted shares | Tranche 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period per tranche (in years)	2 years
Subsequent holding period per tranche (in years)	2 years
2016 Restricted granted shares | Tranche 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period per tranche (in years)	2 years
Subsequent holding period per tranche (in years)	1 year
2016 Restricted granted shares | Named Executive Officers (NEOs) | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded (in shares)	73,474	[1]
2016 Restricted granted shares | Named Executive Officers (NEOs) | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded (in shares)	110,223	[1]
2016 Restricted granted shares | Leadership Team other than NEOs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded (in shares)	26,683
2015 Restricted granted shares | Tranche 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period per tranche (in years)	2 years
Subsequent holding period per tranche (in years)	3 years
2015 Restricted granted shares | Tranche 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period per tranche (in years)	2 years
Subsequent holding period per tranche (in years)	2 years
2015 Restricted granted shares | Tranche 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period per tranche (in years)	2 years
Subsequent holding period per tranche (in years)	1 year
2015 Restricted granted shares | Named Executive Officers (NEOs) | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded (in shares)	58,200	[1]
2015 Restricted granted shares | Named Executive Officers (NEOs) | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded (in shares)	101,850	[1]
2015 Restricted granted shares | Leadership Team other than NEOs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded (in shares)	28,378

[1] The named executive officers are awarded a range of awards that may be earned based on attainment of goals between a target level of performance and a maximum level of performance.