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Retirement Benefit Plans | 3 Months Ended
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Sep. 30, 2019
Retirement Benefit Plans |
Retirement Benefit Plans | 8. Retirement Benefit Plans ​ We sponsor various retirement benefit plans including qualified and nonqualified defined benefit pension plans for our employees. The components of net periodic pension expense are as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended September 30,
​ |  | 2018 | ​ | 2019
Service cost | ​ | $ | 98 | ​ | $ | 100
Interest cost | ​ | ​ | 8 | ​ | ​ | 8
Amortization of prior service cost | ​ |  | 14 | ​ |  | 13
Net periodic pension expense | ​ | $ | 120 | ​ | $ | 121
​ During each of the three months ended September 30, 2018 and 2019, we made no contributions to these defined benefit plans. ​ We also maintain various defined contribution plans. During each of the three month periods ended September 30, 2018 and 2019, we made contributions totaling $1.8 million to these defined contribution plans.

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- Definition The entire disclosure for pension and other postretirement benefits. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -URI http://asc.fasb.org/topic&trid=2235017 + Details
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