Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Differences between our effective tax rate for financial statement purposes and the Federal statutory rate
Statutory tax rate									35.00%	35.00%	35.00%
Federal tax at statutory rate									$ 231,643	$ 200,448	$ 194,049
Nontaxable municipal bond interest and dividend received deduction									(35,166)	(34,734)	(31,939)
State income taxes, net of federal tax benefit									2,135	2,097	3,619
Foreign income taxes									42,093	42,691	40,703
Foreign tax credits									(42,093)	(42,691)	(40,703)
Uncertain tax positions (net of federal tax benefit (expense) on state positions: $223 in 2014, $340 in 2013 and $719 in 2012)									1,091	518	878
Other, net									3,790	(2,816)	(3,420)
Income tax expense	$ 51,534	$ 64,390	$ 40,508	$ 47,061	$ 41,373	$ 41,925	$ 36,669	$ 45,546	$ 203,493	$ 165,513	$ 163,187
Effective tax rate									30.70%	28.90%	29.40%