Business Acquisition (Details 1) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Business Acquisition, Pro Forma Information [Abstract]
Premium revenue	$ 2,446,332	$ 1,866,228	$ 6,858,713	$ 5,404,580
Net income	$ 32,143	$ 58,076	$ 96,191	$ 171,716
Basic earnings per share	$ 0.67	$ 1.22	$ 2.03	$ 3.57
Diluted earnings per share	$ 0.63	$ 1.16	$ 1.86	$ 3.31