Defined Pension Plans (Schedule of Net Periodic Pension Costs) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Defined Benefit Pension Plan [Member]
Service cost - benefits earned during the year	$ 3,519	$ 2,799
Interest cost on projected benefit obligation	2,821	2,570
Expected return on plan assets	(4,472)	(3,967)
Net amortization and deferral	1,849	1,473
Net periodic pension cost	3,717	2,875
Unfunded Supplemental Executive Retirement Plan [Member]
Service cost - benefits earned during the year	144	115
Interest cost on projected benefit obligation	335	307
Net amortization and deferral	551	360
Net periodic pension cost	$ 1,030	$ 782