12. Employee benefit plans

Defined Contribution Plans

We have a defined contribution plan qualified under Section 401(k) of the Internal Revenue Code covering all U.S. employees. Employees may contribute a portion of their compensation, which is then matched by us, subject to certain limitations. Defined contribution expense for the three and six months ended June 30, 2018 was $2.8 million and $5.5 million, respectively. Defined contribution expense for the three and six months ended June 30, 2017 was $2.3 million and $4.5 million, respectively.

Defined Benefit Pension Plans

We have defined benefit pension plans for our employees in Europe which provide benefits to employees upon retirement, death or disability.

The net periodic benefit cost was as follows (in thousands):


	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2018		2017		2018		2017
Service cost	$	1,032	$	680	$	2,111	$	1,296
Interest cost		68		52		139		98
Expected return on plan assets		(47)		(38)		(97)		(73)
Amortization of prior service cost		45		13		90		25
Amortization of actuarial losses		66		34		132		68
Net periodic benefit cost	$	1,164	$	741	$	2,375	$	1,414

The components of net periodic benefit cost other than the service cost component are included in other income (expense), net on the condensed consolidated statements of operations. We expect to contribute a total of $3.2 million to the plans in 2018 inclusive of the amounts contributed to the plan during the current period. As of June 30, 2018 and December 31, 2017, $13.7 million and $13.4 million, respectively, of accrued pension obligation is recorded in other long term liabilities on the condensed consolidated balance sheets.