Investment in Securities (Schedule of the Fair Values of Securities with Unrealized Losses) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Estimated fair value, less than 12 months	$ 74,377	$ 19,758
Unrealized losses, less than 12 months	2,828	112
Estimated fair value, 12 months or more
Unrealized losses, 12 months or more
Estimated fair value, total	74,377	19,758
Unrealized losses, total	2,828	112
State and Political Subdivisions [Member]
Estimated fair value, less than 12 months	74,377	19,758
Unrealized losses, less than 12 months	2,828	112
Estimated fair value, 12 months or more
Unrealized losses, 12 months or more
Estimated fair value, total	74,377	19,758
Unrealized losses, total	2,828	112
Certificates Of Deposit [Member]
Estimated fair value, less than 12 months
Unrealized losses, less than 12 months
Estimated fair value, 12 months or more
Unrealized losses, 12 months or more
Estimated fair value, total
Unrealized losses, total