Investments (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Available-for-sale securities	$ 4,400,000		$ 4,400,000		$ 2,400,000
Net change in unrealized gains (losses) from other trading account assets	200,000	(1,100,000)	(300,000)	(3,900,000)
Net investment income	71,371,000	72,987,000	214,441,000	230,705,000
Gross unrealized losses related to high or highest quality securities	1,500,000		1,500,000		5,400,000
Gross unrealized losses related to other than high or highest quality securities	1,100,000		1,100,000		3,800,000
Gross unrealized losses representing declines in value of greater than 20%					3,400,000
Gross unrealized losses in a continuous loss position for less than six months					300,000
Twelve months or more Unrealized Losses	1,603,000		1,603,000		4,348,000
Asset Backed Securities and Manufacturing [Member]
Twelve months or more Unrealized Losses	1,600,000		1,600,000		4,300,000
Services [Member]
Twelve months or more Unrealized Losses					1,700,000
Manufacturing [Member]
Twelve months or more Unrealized Losses					$ 1,100,000