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Employee benefit plans - Net periodic benefit cost for defined benefit plans (Detail) - JPY (¥)
¥ in Millions
12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2015
Employee benefit plans      
Service cost ¥ 8,909 ¥ 8,253 ¥ 7,800
Interest cost 1,444 2,092 3,090
Expected return on plan assets (6,004) (6,064) (5,732)
Amortization of net actuarial losses 2,867 1,456 2,127
Amortization of prior service cost (1,090) (1,148) (1,148)
Net periodic benefit cost ¥ 6,126 ¥ 4,589 ¥ 6,137