Income Taxes - Reconciliation of Tax Expense at Effective Tax Rate (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Effective income tax rate reconciliation, at federal statutory income tax rate	35.00%	35.00%	35.00%
Income Tax Expense, Continuing Operations Income Tax Reconciliation [Abstract]
Tax expense at the statutory tax rate	$ 45,365	$ 46,446	$ 30,892
Increase (decrease) in tax resulting from:
Tax-exempt income	(4,255)	(3,620)	(3,498)
State income tax, net of federal income tax benefit	3,541	3,741	2,524
Other, net	563	(1)	120
Total income tax expense	$ 45,214	$ 46,566	$ 30,038