Escrow Deposits, Like-Kind Exchange Deposits and Trust Assets (Detail) - USD ($) $ in Billions	Sep. 30, 2015	Dec. 31, 2014
Subsidiaries And Third Party Financial Institutions
Other Assets [Line Items]
Escrow deposits	$ 8.0	$ 6.3
First American Trust
Other Assets [Line Items]
Escrow deposits	2.5	2.2
Trust assets	3.0	3.0
Third Party Financial Institutions
Other Assets [Line Items]
Like-kind exchange funds	$ 2.9	$ 2.4