Other Employee Benefits - Schedule of Shares Purchased in Open Market for Employees (Details) - USD ($)													12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	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, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense													$ 5,912,000	$ 5,369,000	$ 4,024,000
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares	6,822	4,939	5,443	4,322	3,931	5,246	5,180	4,961	4,556	4,226	4,726	4,760	21,526	19,318	18,268
Common Stock Fair Value	$ 18.61	$ 23.45	$ 25.72	$ 29.98	$ 36.87	$ 26.99	$ 34.38	$ 35.08	$ 43.47	$ 37.80	$ 37.16	$ 34.89	$ 23.80	$ 33.06	$ 38.29
Purchase Price	$ 15.82	$ 19.93	$ 21.86	$ 25.49	$ 31.34	$ 22.94	$ 29.22	$ 29.82	$ 36.95	$ 32.13	$ 31.59	$ 29.66	$ 20.23	$ 28.10	$ 32.55
Compensation Expense	$ 19,033	$ 17,385	$ 21,010	$ 19,406	$ 21,738	$ 21,246	$ 26,729	$ 26,095	$ 29,705	$ 23,961	$ 26,324	$ 24,895	$ 76,834	$ 95,808	$ 104,885