Fair value measurements Fair Value Measurements on a Nonrecurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	$ 4,578,822	$ 4,211,290
Level 3 | Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	4,578,822	4,211,290
Fair value measurements on a nonrecurring basis | Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	2,000	4,000
Tax credit investments	9,000
Real estate acquired in settlement of loans	0	0
Fair value measurements on a nonrecurring basis | Level 3
Fair value measurements on a recurring and nonrecurring basis
Loans	2,000	4,000
Tax credit investments	9,000
Real estate acquired in settlement of loans	$ 0	$ 0