Condensed consolidating financial information - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operations:
Cash provided by (used in) operating activities	$ (20,819)	$ 78,315	$ (3,429)
Cash flows from investing activities
Purchase of office facilities	(5,889)	(4,398)	(14,012)
Cash used in investing activities	(5,889)	(4,398)	(14,012)
Cash flows from financing activities
Cash dividends paid on Class A non-voting and Class B voting common stock	(6,008)	(5,983)	(5,978)
Issuance of Class A non-voting common stock	0	185	150
Repurchase of Class A non-voting common stock for cancellation	(8,250)	0	(3,625)
Tax (deficiency) benefit from share-based awards	(277)	1,194	(78)
Redemption of senior secured notes	0	(45,000)	0
Increase in bank call loans, net	40,800	(58,800)	(10,100)
Cash provided by (used in) financing activities	26,265	(108,404)	(19,631)
Net decrease in cash and cash equivalents	(443)	(34,487)	(37,072)
Cash and cash equivalents, beginning of year	63,807	98,294	135,366
Cash and cash equivalents, end of year	63,364	63,807	98,294
Eliminations
Cash flows from operations:
Cash provided by (used in) operating activities	0	0	0
Cash flows from investing activities
Purchase of office facilities	0	0	0
Cash used in investing activities	0	0	0
Cash flows from financing activities
Cash dividends paid on Class A non-voting and Class B voting common stock	0	0	0
Issuance of Class A non-voting common stock		0	0
Repurchase of Class A non-voting common stock for cancellation	0		0
Tax (deficiency) benefit from share-based awards	0	0	0
Redemption of senior secured notes		0
Increase in bank call loans, net	0	0	0
Cash provided by (used in) financing activities	0	0	0
Net decrease in cash and cash equivalents	0	0	0
Cash and cash equivalents, beginning of year	0	0	0
Cash and cash equivalents, end of year	0	0	0
Parent | Reportable Legal Entities
Cash flows from operations:
Cash provided by (used in) operating activities	15,003	49,595	9,944
Cash flows from investing activities
Purchase of office facilities	0	0	0
Cash used in investing activities	0	0	0
Cash flows from financing activities
Cash dividends paid on Class A non-voting and Class B voting common stock	(6,008)	(5,983)	(5,978)
Issuance of Class A non-voting common stock		185	150
Repurchase of Class A non-voting common stock for cancellation	(8,250)		(3,625)
Tax (deficiency) benefit from share-based awards	(277)	1,194	(78)
Redemption of senior secured notes		(45,000)
Increase in bank call loans, net		0	0
Cash provided by (used in) financing activities	(14,535)	(49,604)	(9,531)
Net decrease in cash and cash equivalents	468	(9)	413
Cash and cash equivalents, beginning of year	439	448	35
Cash and cash equivalents, end of year	907	439	448
Guarantor Subsidiaries | Reportable Legal Entities
Cash flows from operations:
Cash provided by (used in) operating activities	1,029	(29,344)	(9,757)
Cash flows from investing activities
Purchase of office facilities	0	0	0
Cash used in investing activities	0	0	0
Cash flows from financing activities
Cash dividends paid on Class A non-voting and Class B voting common stock	0	0	0
Issuance of Class A non-voting common stock		0	0
Repurchase of Class A non-voting common stock for cancellation	0		0
Tax (deficiency) benefit from share-based awards	0	0	0
Redemption of senior secured notes		0
Increase in bank call loans, net	0	0	0
Cash provided by (used in) financing activities	0	0	0
Net decrease in cash and cash equivalents	1,029	(29,344)	(9,757)
Cash and cash equivalents, beginning of year	1,557	30,901	40,658
Cash and cash equivalents, end of year	2,586	1,557	30,901
Non-Guarantor Subsidiaries | Reportable Legal Entities
Cash flows from operations:
Cash provided by (used in) operating activities	(36,851)	58,064	(3,616)
Cash flows from investing activities
Purchase of office facilities	(5,889)	(4,398)	(14,012)
Cash used in investing activities	(5,889)	(4,398)	(14,012)
Cash flows from financing activities
Cash dividends paid on Class A non-voting and Class B voting common stock	0	0	0
Issuance of Class A non-voting common stock		0	0
Repurchase of Class A non-voting common stock for cancellation	0		0
Tax (deficiency) benefit from share-based awards	0	0	0
Redemption of senior secured notes		0
Increase in bank call loans, net	40,800	(58,800)	(10,100)
Cash provided by (used in) financing activities	40,800	(58,800)	(10,100)
Net decrease in cash and cash equivalents	(1,940)	(5,134)	(27,728)
Cash and cash equivalents, beginning of year	61,811	66,945	94,673
Cash and cash equivalents, end of year	$ 59,871	$ 61,811	$ 66,945