Estimated Fair Value of Financial Instruments (Information about Carrying Amount of Financial Instruments Reported in Consolidated Balance Sheets and Related Market or Fair Value) (Parenthetical) (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2018		Mar. 31, 2017
Carrying amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Not practicable to estimate fair value	¥ 140,155	[1]	¥ 149,820	[2]
Carrying amount | Investment in securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Not practicable to estimate fair value	140,155		149,820
Estimated fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Not practicable to estimate fair value	140,155	[1]	149,820	[2]
Estimated fair value | Investment in securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Not practicable to estimate fair value	¥ 140,155		¥ 149,820

[1] The fair value of investment securities of ¥140,155 million was not estimated, as it was not practicable.

[2] The fair value of investment securities of ¥149,820 million was not estimated, as it was not practicable.