Related Party Transactions	12 Months Ended
Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions Weichai Transactions See Note 3. Weichai Transactions for information regarding the Weichai SPA and collaboration arrangement. 3PI Lease Agreement In connection with the acquisition of 3PI, the Company entered into a lease agreement effective April 1, 2014, with a limited liability company in which one of the former owners of 3PI is the sole member. The lease is for the land, buildings and certain equipment located at 3PI’s facilities in Darien, Wisconsin. The lease expires on March 31, 2021. In 2017, 2016, 2015 and 2014, the Company recognized expense of $0.5 million, $0.5 million, $0.5 million and $0.4 million, respectively, in connection with this lease. See Note 5. Acquisitions for additional information on this acquisition. The former 3PI owner is no longer employed by the Company and is no longer a related party. Transactions with Joint Ventures MAT-PSI Holdings, LLC On December 7, 2012, the Company and MAT Holdings, Inc. (“MAT”) entered into an agreement to create MAT-PSI Holdings, LLC (“MAT-PSI”), which was intended to be a holding company of its 100% Chinese wholly-owned foreign entity, referred to as Green Power. The Company invested $0.9 million for its 50% share of MAT-PSI, which was formed to manufacture, assemble and supply natural gas, gas and alternative-fueled power systems to Chinese and Asian forklift customers. The venture established a production facility in Dalian and also sourced base engines from a local Chinese factory. As MAT-PSI was not profitable, the venture was closed in 2017. The Company’s investment was accounted for under the equity method of accounting. Doosan-PSI, LLC In 2015, the Company and Doosan Infracore Co., Ltd. (“Doosan”), a subsidiary of Doosan Group, entered into an agreement to form Doosan-PSI, LLC. The Company invested $1.0 million to acquire 50% of the venture, which was formed to operate in the field of developing, designing, testing, manufacturing, assembling, branding, marketing, selling, distributing and providing support for industrial gas engines and all components and materials required for assembly of the gas engines to the global power generation market outside of North America and South Korea. Sterling and Wilson Cogen Solutions, LLC On April 13, 2016, the Company and Sterling and Wilson Power Systems Inc. (“SW”) entered into an agreement for the formation and operation of Sterling and Wilson Cogen Solutions, LLC. The Company invested $0.2 million for its 49% share of the venture. Effective May 31, 2017, the Company sold its 49% membership interest in the venture to SW for $0.2 million pursuant to the agreement. Joint Venture Operating Results The Company's Consolidated Statements of Operations included losses of $0.4 million, $0.1 million and a gain of $0.3 million presented in “Other income (expense), net” in 2016, 2015 and 2014, respectively, for these ventures. In 2017, the gain was immaterial. Transactions with Related Individuals William D. Winemaster William D. Winemaster, the father of Gary S. Winemaster, former Chairman of the Board, Chief Executive Officer and President and nonexecutive Chief Strategy Officer, and Kenneth J. Winemaster, Executive Vice President, served as an employee performing consulting and advisory-type services, for which he was paid a salary, an amount for automobiles and related insurance premiums, mobile-telephone services and a bonus. The Company paid to William D. Winemaster $0.2 million in each of the years 2017, 2016, 2015 and 2014.

Related Party Transactions

Weichai Transactions

See Note 3. Weichai Transactions for information regarding the Weichai SPA and collaboration arrangement.

3PI Lease Agreement

In connection with the acquisition of 3PI, the Company entered into a lease agreement effective April 1, 2014, with a limited liability company in which one of the former owners of 3PI is the sole member. The lease is for the land, buildings and certain equipment located at 3PI’s facilities in Darien, Wisconsin. The lease expires on March 31, 2021. In 2017, 2016, 2015 and 2014, the Company recognized expense of $0.5 million, $0.5 million, $0.5 million and $0.4 million, respectively, in connection with this lease. See Note 5. Acquisitions for additional information on this acquisition. The former 3PI owner is no longer employed by the Company and is no longer a related party.

Transactions with Joint Ventures

MAT-PSI Holdings, LLC

On December 7, 2012, the Company and MAT Holdings, Inc. (“MAT”) entered into an agreement to create MAT-PSI Holdings, LLC (“MAT-PSI”), which was intended to be a holding company of its 100% Chinese wholly-owned foreign entity, referred to as Green Power. The Company invested $0.9 million for its 50% share of MAT-PSI, which was formed to manufacture, assemble and supply natural gas, gas and alternative-fueled power systems to Chinese and Asian forklift customers. The venture established a production facility in Dalian and also sourced base engines from a local Chinese factory. As MAT-PSI was not profitable, the venture was closed in 2017. The Company’s investment was accounted for under the equity method of accounting.

Doosan-PSI, LLC

In 2015, the Company and Doosan Infracore Co., Ltd. (“Doosan”), a subsidiary of Doosan Group, entered into an agreement to form Doosan-PSI, LLC. The Company invested $1.0 million to acquire 50% of the venture, which was formed to operate in the field of developing, designing, testing, manufacturing, assembling, branding, marketing, selling, distributing and providing support for industrial gas engines and all components and materials required for assembly of the gas engines to the global power generation market outside of North America and South Korea.

Sterling and Wilson Cogen Solutions, LLC

On April 13, 2016, the Company and Sterling and Wilson Power Systems Inc. (“SW”) entered into an agreement for the formation and operation of Sterling and Wilson Cogen Solutions, LLC. The Company invested $0.2 million for its 49% share of the venture. Effective May 31, 2017, the Company sold its 49% membership interest in the venture to SW for $0.2 million pursuant to the agreement.

Joint Venture Operating Results

The Company's Consolidated Statements of Operations included losses of $0.4 million, $0.1 million and a gain of $0.3 million presented in “Other income (expense), net” in 2016, 2015 and 2014, respectively, for these ventures. In 2017, the gain was immaterial.

Transactions with Related Individuals

William D. Winemaster

William D. Winemaster, the father of Gary S. Winemaster, former Chairman of the Board, Chief Executive Officer and President and nonexecutive Chief Strategy Officer, and Kenneth J. Winemaster, Executive Vice President, served as an employee performing consulting and advisory-type services, for which he was paid a salary, an amount for automobiles and related insurance premiums, mobile-telephone services and a bonus. The Company paid to William D. Winemaster $0.2 million in each of the years 2017, 2016, 2015 and 2014.