Investments in debt and equity securities available-for-sale - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized costs
Debt securities	$ 650,422	$ 665,646
Fair values
Debt securities	645,768	671,441
Municipal
Amortized costs
Debt securities	63,187	71,581
Fair values
Debt securities	63,123	72,669
Corporate
Amortized costs
Debt securities	347,713	351,477
Fair values
Debt securities	345,966	357,933
Foreign
Amortized costs
Debt securities	226,683	229,750
Fair values
Debt securities	224,217	228,237
U.S. Treasury Bonds
Amortized costs
Debt securities	12,839	12,838
Fair values
Debt securities	$ 12,462	$ 12,602