Acquisition (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Mar. 31, 2011 National Interstate (NATL) [Member]	Jul. 31, 2010 National Interstate (NATL) [Member]	Dec. 31, 2010 Vanliner Group, Inc. Moving and Storage Insurance Business [Member]
Acquisition (Textuals) [Abstract]
Percentage ownership of National Interstate		52.00%
Purchase of subsidiaries including post-closing adjustments	$ 114
Gross written premiums			$ 90
Percentage of total gross written premiums of Vanliner Group, Inc.			78.00%